Fix synthetic mouse moves in Ozone X11.

When a mouse button is pressed an EnterNotify / LeaveNotify XEvent is
generated for the nested XWindow used by the GPU process. The
EnterNotify ends up becoming a ET_MOUSE_DRAGGED event which is wrong. In
general, we want to just ignore the EnterNotify / LeaveNotify events for
nested XWindows.

Bug: 792322
Change-Id: I59af57199c0567001256026332823350b4a45d9e
Reviewed-on: https://chromium-review.googlesource.com/819595Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523118}

ui/events/platform/x11/x11_event_source_libevent.cc

@@ -97,6 +97,11 @@ std::unique_ptr<ui::Event> TranslateXEventToEvent(const XEvent& xev) {
   switch (xev.type) {
     case LeaveNotify:
     case EnterNotify:
+      // Don't generate synthetic mouse move events for EnterNotify/LeaveNotify
+      // from nested XWindows. https://crbug.com/792322
+      if (xev.xcrossing.detail == NotifyInferior)
+        return nullptr;
+
       return std::make_unique<MouseEvent>(EventTypeFromXEvent(xev),
                                           EventLocationFromXEvent(xev),
                                           EventSystemLocationFromXEvent(xev),