* fix: re-enable MacWebContentsOcclusion with embedder window fix

Replace the full revert of Chromium's MacWebContentsOcclusion cleanup
with a targeted patch that handles embedder windows shown after
WebContentsViewCocoa attachment. This lets us drop the feature flag
disable in feature_list.cc and re-enable upstream occlusion tracking.

Adds tests for show/hide event counts on macOS and visibility tracking
across multiple child WebContentsViews.

* test: drop show/hide event count assertion

The assertion that 'show' fires exactly once per w.show() call is not
an API guarantee - macOS can send multiple occlusion state
notifications during a single show() when other windows are on screen
(common on CI after hundreds of prior tests). The
visibilitychange-count test in api-web-contents-view-spec.ts covers
the actual invariant we care about.

* fix: ignore WebContentsOcclusionCheckerMac synthetic notifications in window delegate

On macOS 13.3-25.x, Chromium's occlusion checker enables manual
frame-intersection detection and posts synthetic
NSWindowDidChangeOcclusionStateNotification tagged with its class name
in userInfo. These fire when the checker's NSContainsRect heuristic
decides a window is covered by another window's frame, but the real
-[NSWindow occlusionState] hasn't changed.

Our delegate was treating these the same as real macOS notifications
and emitting show/hide events based on occlusionState, which was
unchanged - resulting in spurious duplicate show events when e.g.
Quick Look opened and its frame intersected the BrowserWindow.