8eddc4af8d2cac8e2d797c1b9b6d37f376f48577  Add element tagging for ad popup label detection

Classified as not security-relevant.
Triage note: Adds is_overlay_popup_ad_ flag and threads Element* through OnPopupDetected for the ad highlight label (Bug 567066858). Feature plumbing; no 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: security_assert (+3.0), null_guard (+1.2), area:third_party/blink/renderer/core/ (+2.5)