Prevent possible crash unregistering VR BroadcastReceiver

I only saw this crash once, and can't reproduce, but better safe than sorry.

BUG=712823

Review-Url: https://codereview.chromium.org/2830463002
Cr-Commit-Position: refs/heads/master@{#465702}
(cherry picked from commit 3eb6c9f)

Review-Url: https://codereview.chromium.org/2826213002 .
Cr-Commit-Position: refs/branch-heads/3071@{#62}
Cr-Branched-From: a106f0a-refs/heads/master@{#464641}

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

@@ -157,7 +157,11 @@ public void onReceive(Context context, Intent intent) {
         public void unregister() {
             ChromeActivity activity = mTargetActivity.get();
             if (activity == null) return;
-            activity.unregisterReceiver(VrBroadcastReceiver.this);
+            try {
+                activity.unregisterReceiver(VrBroadcastReceiver.this);
+            } catch (IllegalArgumentException e) {
+                // Ignore this. This means our receiver was already unregistered somehow.
+            }
         }
     }