613afab6d7a2c9ff3b3a747c9ffe7f91a81b321d  [Site Isolation] Stop button never turns back into reload button on certain pages

Classified as not security-relevant.
Triage note: Adds Document::checkLoadComplete and calls it (alongside checkCompleted) from RemoteFrame::didFinishLoadInAnotherProcess so a site-isolated child frame finishing in another process also drives FrameLoader::checkLoadComplete; a load-event completion correctness/plumbing change, not a memory-safety or security bug.
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 (+4.87), area:Source/WebCore/dom (+2.0), surgical (+1.3)