Revert "Disable several flaky WebVrTransitionTests."

This reverts commit 88aa3d56.

Reason for revert: These flakes are addressed with  https://chromium.googlesource.com/chromium/src.git/+/044cca4ad8bc7a80c6edbd4e8dd4d078fd5f7659

Original change's description:
> Disable several flaky WebVrTransitionTests.
> 
> TBR=bsheedy@chromium.org
> 
> Bug: 840638
> Change-Id: I3acc6d00d75f6c6dce370961d7459dc3a50faef4
> Reviewed-on: https://chromium-review.googlesource.com/1049274
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#556686}

TBR=bsheedy@chromium.org,jbudorick@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 840638
Change-Id: I46f8ffa148d1944ff4f05241fd93e2220fbd93ae
Reviewed-on: https://chromium-review.googlesource.com/1054119
Commit-Queue: Eric Karl <ericrk@chromium.org>
Reviewed-by: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557705}

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

@@ -37,7 +37,6 @@ import org.chromium.base.test.params.ParameterAnnotations.UseRunnerDelegate;
 import org.chromium.base.test.params.ParameterSet;
 import org.chromium.base.test.params.ParameterizedRunner;
 import org.chromium.base.test.util.CommandLineFlags;
-import org.chromium.base.test.util.DisabledTest;
 import org.chromium.base.test.util.MinAndroidSdkLevel;
 import org.chromium.base.test.util.Restriction;
 import org.chromium.base.test.util.RetryOnFailure;
@@ -364,7 +363,6 @@ public class WebVrTransitionTest {
      */
     @Test
     @MediumTest
-    @DisabledTest(message = "crbug.com/840638")
     public void testControlsVisibleAfterExitingVr() throws InterruptedException {
         controlsVisibleAfterExitingVrImpl(
                 VrTestFramework.getFileUrlForHtmlTestFile("generic_webvr_page"), mVrTestFramework);
@@ -508,7 +506,6 @@ public class WebVrTransitionTest {
      */
     @Test
     @MediumTest
-    @DisabledTest(message = "crbug.com/840638")
     @CommandLineFlags.Remove({"enable-webvr"})
     @CommandLineFlags.Add({"enable-features=WebXR"})
     @VrActivityRestriction({VrActivityRestriction.SupportedActivity.ALL})