99127ce297d9945557256245abee97d0d42d4ce8  [iOS] Media controls context menus can appear at the wrong position with Site Isolation enabled

Classified as not security-relevant.
Triage note: Adds contextMenuAnchorRect()/mediaControlsContextMenuAnchorRectForBindings using convertToRootViewAcrossIsolatedFrames and a new IDL binding. Feature/plumbing for correct menu positioning under site isolation; no memory-safety or logic bug fixed.
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 (+1.95), surgical (+1.3)