Added comments in the WindowEventDispatcher::OnOtherRootGotCapture function.

This is to clarify why we should not be dispatching synthetic mouse exit events on
Windows.

BUG=394672
R=sky

Review URL: https://codereview.chromium.org/454053004

Cr-Commit-Position: refs/heads/master@{#288970}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288970 0039d316-1c4b-4281-b951-d872f2087c98

ui/aura/window_event_dispatcher.cc

@@ -391,8 +391,12 @@ void WindowEventDispatcher::UpdateCapture(Window* old_capture,
 }
 
 void WindowEventDispatcher::OnOtherRootGotCapture() {
-  // Sending the mouse exit causes bugs on Windows (e.g. crbug.com/394672).
-  // TODO(pkotwicz): Fix the bugs and send mouse exit on Windows too.
+  // Windows provides the TrackMouseEvents API which allows us to rely on the
+  // OS to send us the mouse exit events (WM_MOUSELEAVE). Additionally on
+  // desktop Windows, every top level window could potentially have its own
+  // root window, in which case this function will get called whenever those
+  // windows grab mouse capture. Sending mouse exit messages in these cases
+  // causes subtle bugs like (crbug.com/394672).
 #if !defined(OS_WIN)
   if (mouse_moved_handler_) {
     // Dispatch a mouse exit to reset any state associated with hover. This is