Make window cloaking and uncloaking trigger occlusion calculation.

This is the right thing to do in general, since we treat cloaked windows
as non-occluding. It also fixes an issue with switching virtual desktops
since that triggers window cloaking and uncloaking events.

Bug: 1137419
Change-Id: I2b66182bc72c17e69c3f72cd793e2924ba483bb9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2478082
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#818031}

ui/aura/native_window_occlusion_tracker_win.cc

@@ -519,6 +519,10 @@ void NativeWindowOcclusionTrackerWin::WindowOcclusionCalculator::
   // maximize and native window restore events.
   RegisterGlobalEventHook(EVENT_OBJECT_STATECHANGE, EVENT_OBJECT_STATECHANGE);
 
+  // Cloaking and uncloaking of windows should trigger an occlusion calculation.
+  // In particular, switching virtual desktops seems to generate these events.
+  RegisterGlobalEventHook(EVENT_OBJECT_CLOAKED, EVENT_OBJECT_UNCLOAKED);
+
   // Determine which subset of processes to set EVENT_OBJECT_LOCATIONCHANGE on
   // because otherwise event throughput is very high, as it generates events
   // for location changes of all objects, including the mouse moving on top of a