e5b8d50e12553b0bb1a8ff9e8116d060028a7228  Make JavaScript wrapper around contentsToMainFrameView exposed in worlds with autofill JS enabled

Classified as not security-relevant.
Triage note: Adds DOMWindow::convertRectToMainFrameCoordinates gated [EnabledForWorld=allowAutofill], returning view->contentsToMainFrameView(rect). New internal (non-web-exposed) coordinate-conversion helper for autofill UI; feature plumbing, no memory-safety or logic bug addressed.
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 (+2.59), msg_feature (-1.0), surgical (+1.3)