views: enable FocusManagerTest.WidgetFocusChangeListener for mus

It no longer fails.

BUG=664261
TEST=test only change

Change-Id: Id17c9b0950fd20e001b27f104912542daaf8bc8b
Reviewed-on: https://chromium-review.googlesource.com/1213931Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589603}

ui/views/focus/focus_manager_unittest.cc

@@ -130,12 +130,6 @@ TEST_F(FocusManagerTest, FocusChangeListener) {
 }
 
 TEST_F(FocusManagerTest, WidgetFocusChangeListener) {
-  // TODO: this test ends up calling focus on the aura::Window associated with
-  // the Widget and expecting that to change activation. This should work for
-  // aura-mus-client as well. http://crbug.com/664261.
-  if (IsMus())
-    return;
-
   // First, ensure the simulator is aware of the Widget created in SetUp() being
   // currently active.
   test::WidgetTest::SimulateNativeActivate(GetWidget());