Disable flaky WebVrTransitionTest#testWindowRafStopsFiringWhilePresenting on Android

TBR=bsheedy@chromium.org

Bug: 840725
Change-Id: Id47604a849bcfcbb51e33ddf94ffa7a539ecb1df
Reviewed-on: https://chromium-review.googlesource.com/1049668Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556762}

chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/WebVrTransitionTest.java

@@ -406,6 +406,7 @@ public class WebVrTransitionTest {
      */
     @Test
     @MediumTest
+    @DisabledTest(message = "crbug.com/840725")
     @VrActivityRestriction({VrActivityRestriction.SupportedActivity.ALL})
     @RetryOnFailure
     public void testWindowRafStopsFiringWhilePresenting() throws InterruptedException {