[UI-side compositing] Plumb mouseMovedInContentArea event to UI process
<rdar://problem/107732708>
Pull request: https://github.com/WebKit/WebKit/pull/12487
Committed 262699@main (90a47db0ffa4): <https://commits.webkit.org/262699@main> Reviewed commits have been landed. Closing PR #12487 and removing active labels.