VR: Don't attempt to cancel VR entry if we're already in VR.

Headset NFC scan can pause and resume (without stopping) us while
we're already in VR, so we stay in VR while this is happening.
However, when we get paused we want to cancel VR entry so we don't
enter VR while paused. This CL just adds a check to see if we're in
VR before attempting to cancel entry.

Bug: 742509
Change-Id: Ib0cdac3c7e10e5206410835f949f318c73e9349f
Reviewed-on: https://chromium-review.googlesource.com/570742Reviewed-by: Yash Malik <ymalik@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486769}

chrome/android/java/src/org/chromium/chrome/browser/vr_shell/VrShellDelegate.java

@@ -1066,7 +1066,7 @@ public class VrShellDelegate implements ApplicationStatus.ActivityStateListener,
 
         if (mInVr) mVSyncEstimator.pause();
 
-        cancelPendingVrEntry();
+        if (!mInVr) cancelPendingVrEntry();
 
         // When the active web page has a vrdisplayactivate event handler,
         // mListeningForWebVrActivate should be set to true, which means a vrdisplayactive event