[Picture-in-Picture] Fix crash when closing window.

This CL makes sure the active_session_ object exists before shutting
it down. Without this, it was potentially causing a browser crash.

Bug: 1066768
Change-Id: I2793be0862b7c14b957712740c0bbde2b8d9f21a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2131627
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755427}

content/browser/picture_in_picture/picture_in_picture_window_controller_impl.cc

@@ -314,7 +314,9 @@ void PictureInPictureWindowControllerImpl::OnLeavingPictureInPicture(
   }
 
   if (media_player_id_.has_value()) {
-    active_session_->Shutdown();
+    if (active_session_)
+      active_session_->Shutdown();
+
     active_session_ = nullptr;
     media_player_id_.reset();
   }