views/mus: Install a mus-friend ContextFactory for tests.

ui/views/view_unittest.cc

Index: ui/views/view_unittest.cc
diff --git a/ui/views/view_unittest.cc b/ui/views/view_unittest.cc
index 55445de620c0d29beab37b8fcfe49b8c9670b618..62e23be02a3e778dc9ac17ef310439668b3fb473 100644
--- a/ui/views/view_unittest.cc
+++ b/ui/views/view_unittest.cc
@@ -4207,9 +4207,6 @@ class PaintTrackingView : public View {
 // Makes sure child views with layers aren't painted when paint starts at an
 // ancestor.
 TEST_F(ViewLayerTest, DontPaintChildrenWithLayers) {
-  // TODO(sad): DrawWaiterForTest does not work with mus. crbug.com/618136
-  if (IsMus())
-    return;
   PaintTrackingView* content_view = new PaintTrackingView;
   widget()->SetContentsView(content_view);
   content_view->SetPaintToLayer();