Stop observation in WebContentsDestroyed

This is needed to prevent a crash where
WebContentController::AttachTo is called after the
controller is destroyed.

Bug: Internal b/144546163
Bug: Internal b/144861135
Test: Compile cast_shell
Change-Id: I71b05e3f6d404a4044c623a1712650e4f459e85a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1934652Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#718796}

chromecast/browser/webview/cast_app_rpc_instance.cc

@@ -65,6 +65,7 @@ void CastAppRpcInstance::CreateCastAppWindowLink(int platform_view_id,
 
 void CastAppRpcInstance::WebContentsDestroyed() {
   controller_.reset();
+  window_manager_->RemoveObserver(this);
 }
 
 }  // namespace chromecast