Check if activity is finishing / destroyed prior to using fullscreen mgr

Prior to calling into ChromeActivity's getFullScreenManager(), we should
check whether the activity is not finishing or already destroyed to
protect ourselves from IllegalStateException.

Fixed: 1038736
Change-Id: Ic8f25f5e839535031d92055b424cd624db4b2704
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1985168
Auto-Submit: Piotr Bialecki <bialpio@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Piotr Bialecki <bialpio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731688}

chrome/android/java/src/org/chromium/chrome/browser/vr/ArImmersiveOverlay.java

@@ -292,7 +292,7 @@ public class ArImmersiveOverlay
         // The JS app may have put an element into fullscreen mode during the immersive session,
         // even if this wasn't visible to the user. Ensure that we fully exit out of any active
         // fullscreen state on session end to avoid being left in a confusing state.
-        if (mActivity.getActivityTab() != null) {
+        if (mActivity.getActivityTab() != null && !mActivity.isActivityFinishingOrDestroyed()) {
             mActivity.getFullscreenManager().onExitFullscreen(mActivity.getActivityTab());
         }