Increment stat when an occluded window handles a mouse event.

Bug: 813093
Change-Id: I72bb4fc6ff11e1499f606851606b49b6f7e80bab
Reviewed-on: https://chromium-review.googlesource.com/c/1359301Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613437}

tools/metrics/histograms/histograms.xml

@@ -70384,6 +70384,18 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="OccludedWindowMouseEvents" units="events"
+    expires_after="2019-12-01">
+  <owner>davidbienvenu@chromium.org</owner>
+  <owner>fdoray@chromium.org</owner>
+  <summary>
+    Incremented each time a mouse event is handled by a window that is marked as
+    occluded. This number should be 0 or very close to 0. It should be much less
+    than the number of times a window is mark occluded -
+    WindowOcclusionChanged.Occluded.
+  </summary>
+</histogram>
+
 <histogram name="OfflineIndicator.CTR" enum="OfflineIndicatorCTREvent"
     expires_after="2019-06-30">
   <owner>dimich@chromium.org</owner>

ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc

@@ -6,6 +6,7 @@
 
 #include "base/containers/flat_set.h"
 #include "base/memory/ptr_util.h"
+#include "base/metrics/histogram_macros.h"
 #include "third_party/skia/include/core/SkPath.h"
 #include "third_party/skia/include/core/SkRegion.h"
 #include "ui/aura/client/aura_constants.h"
@@ -897,9 +898,13 @@ void DesktopWindowTreeHostWin::HandleNativeBlur(HWND focused_window) {
 }
 
 bool DesktopWindowTreeHostWin::HandleMouseEvent(ui::MouseEvent* event) {
-  // TODO(davidbienvenu): Check for getting mouse events for an occluded window
-  // with either a DCHECK or a stat.  Event can cause this object to be deleted
-  // so look at occlusion state before we do anything with the event.
+  // Mouse events in occluded windows should be very rare. If this stat isn't
+  // very close to 0, that would indicate that windows are incorrectly getting
+  // marked occluded, or getting stuck in the occluded state. Event can cause
+  // this object to be deleted so check occlusion state before we do anything
+  // with the event.
+  if (window()->occlusion_state() == aura::Window::OcclusionState::OCCLUDED)
+    UMA_HISTOGRAM_BOOLEAN("OccludedWindowMouseEvents", true);
   SendEventToSink(event);
   return event->handled();
 }