Picture in Picture: Fix MAC picture-in-picture crash.

Bug: 1111982
Change-Id: I28eb5de0f0a87be0cbdd8922da65af58447f2167
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453438Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814867}

third_party/blink/renderer/modules/picture_in_picture/picture_in_picture_controller_impl.cc

@@ -242,7 +242,9 @@ void PictureInPictureControllerImpl::ExitPictureInPicture(
   if (!EnsureService())
     return;
 
-  DCHECK(picture_in_picture_session_.is_bound());
+  if (!picture_in_picture_session_.is_bound())
+    return;
+
   picture_in_picture_session_->Stop(
       WTF::Bind(&PictureInPictureControllerImpl::OnExitedPictureInPicture,
                 WrapPersistent(this), WrapPersistent(resolver)));