Disable flaky test: MediaGalleriesRead.

The test MediaGalleriesPlatformAppBrowserTest.MediaGalleriesRead is
flaky on MacOSX bots.

BUG=416128
NOTRY=true
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/588663003

Cr-Commit-Position: refs/heads/master@{#295771}

chrome/browser/extensions/api/media_galleries/media_galleries_apitest.cc

@@ -515,8 +515,14 @@ IN_PROC_BROWSER_TEST_F(MediaGalleriesPlatformAppBrowserTest,
   ASSERT_TRUE(RunMediaGalleriesTest("no_galleries_copy_to")) << message_;
 }
 
+// Test is flaky. crbug.com/416128
+#if defined(OS_MACOSX)
+#define MAYBE_MediaGalleriesRead DISABLED_MediaGalleriesRead
+#else
+#define MAYBE_MediaGalleriesRead MediaGalleriesRead
+#endif
 IN_PROC_BROWSER_TEST_F(MediaGalleriesPlatformAppBrowserTest,
-                       MediaGalleriesRead) {
+                       MAYBE_MediaGalleriesRead) {
   RemoveAllGalleries();
   MakeSingleFakeGallery(NULL);
   base::ListValue custom_args;