Implement IsWindowUnderCursor()

MacViewsBrowser, it is necessary to open bookmark menus on mouse
over. So, implement the unimplemented function IsWindowUnderCursor().

MenuController: :ShowSiblingMenu() use IsWindowUnderCursor(). On
Bug: 762764
Change-Id: Ifbf93085a0546f7e3c2b6070267e62212b84bddd
Reviewed-on: https://chromium-review.googlesource.com/654400Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539165}

ui/display/mac/screen_mac.mm

@@ -170,8 +170,8 @@ class ScreenMac : public Screen {
   }
 
   bool IsWindowUnderCursor(gfx::NativeWindow window) override {
-    NOTIMPLEMENTED();
-    return false;
+    return [NSWindow windowNumberAtPoint:[NSEvent mouseLocation]
+             belowWindowWithWindowNumber:0] == [window windowNumber];
   }
 
   gfx::NativeWindow GetWindowAtScreenPoint(const gfx::Point& point) override {