Mark DialogClientViewTest.ButtonPosition flaky on Mac

This patch marks DialogClientViewTest.ButtonPosition flaky on Mac.

TBR=bsep@chromium.org

Bug: 903000
Change-Id: I7d533956e437c143e4d4030dd357553fd4caabf1
Reviewed-on: https://chromium-review.googlesource.com/c/1325592Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606338}

ui/views/window/dialog_client_view_unittest.cc

@@ -453,7 +453,13 @@ TEST_F(DialogClientViewTest, LinkedWidths) {
   EXPECT_EQ(20, boring_view->width());
 }
 
-TEST_F(DialogClientViewTest, ButtonPosition) {
+// TODO(crbug.com/903000): We should not mark GetIndexOf flaky.
+#if defined(OS_MACOSX)
+#define MAYBE_ButtonPosition DISABLED_ButtonPosition
+#else
+#define MAYBE_ButtonPosition ButtonPosition
+#endif
+TEST_F(DialogClientViewTest, MAYBE_ButtonPosition) {
   constexpr int button_row_inset = 13;
   client_view()->SetButtonRowInsets(gfx::Insets(button_row_inset));
   constexpr int contents_height = 37;