21142271a7e1423a5d85c2df22475858e5f9ae48  Setup view transition should flush the update callback queue

Classified as not security-relevant.
Triage note: Replaces an inline queueTask/weakThis lambda with scheduleUpdateCallback()/flushUpdateCallbackQueue() and a Document-owned Vector<Ref<ViewTransition>> queue to match the css-view-transitions spec; no memory-safety or logic bug is fixed, the old path already guarded with weakThis.get().
This commit was flagged by the signal scorer and reviewed by AI (not a human). It is listed only so the triage is auditable; there is no root-cause analysis because this is not a vulnerability fix.

Structural signals: smartptr_member_added (+6.47), null_guard (+1.2), uaf_lifetime (+3.57), area:Source/WebCore/dom (+2.0)