Disable MSE_ExternalClearKey/EncryptedMediaTest.Playback_Multiple_VideoAudio_WebM

Currently MSE_ExternalClearKey/EncryptedMediaTest.Playback_Multiple_VideoAudio_WebM
is flaking on Mac. So I'm disabling it until that can be resolved.

TBR=xhwang@chromium.org
TEST=MSE_ExternalClearKey/EncryptedMediaTest.Playback_Multiple_VideoAudio_WebM

Bug: 835585
Change-Id: Ifb82978c391d99804667c87a3d43ea1539bf9110
Reviewed-on: https://chromium-review.googlesource.com/1042373Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555732}

chrome/browser/media/encrypted_media_browsertest.cc

@@ -572,7 +572,16 @@ IN_PROC_BROWSER_TEST_P(EncryptedMediaTest, Playback_VideoClearAudio_WebM_Opus) {
   TestSimplePlayback("bear-320x240-opus-av_enc-v.webm", kWebMOpusAudioVp9Video);
 }
 
-IN_PROC_BROWSER_TEST_P(EncryptedMediaTest, Playback_Multiple_VideoAudio_WebM) {
+// TODO(xhwang): Test is flaky on Mac. https://crbug.com/835585
+#if defined(MACOS)
+#define MAYBE_Playback_Multiple_VideoAudio_WebM \
+  DISABLED_Playback_Multiple_VideoAudio_WebM
+#else
+#define MAYBE_Playback_Multiple_VideoAudio_WebM \
+  Playback_Multiple_VideoAudio_WebM
+#endif
+IN_PROC_BROWSER_TEST_P(EncryptedMediaTest,
+                       MAYBE_Playback_Multiple_VideoAudio_WebM) {
   if (!IsPlayBackPossible(CurrentKeySystem())) {
     DVLOG(0) << "Skipping test - Playback_Multiple test requires playback.";
     return;