Remove the observer for stopped speech sessions

This shouldn't be necessary; when the last frame is deleted the observer
should remove itself. However, for paranoia's sake, remove the observers
as sessions are closed.

Bug: 1123557
Change-Id: If0109eafee7fd2ba38480849b99ee42bbafe5b61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2424711
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809766}

content/browser/speech/speech_recognition_manager_impl.cc

@@ -449,6 +449,17 @@ void SpeechRecognitionManagerImpl::StopAudioCaptureForSession(int session_id) {
   if (iter == sessions_.end())
     return;
 
+  // The deletion observer is owned by this class, so it's safe to use
+  // Unretained.
+  GetUIThreadTaskRunner({})->PostTask(
+      FROM_HERE,
+      base::BindOnce(&SpeechRecognitionManagerImpl::FrameDeletionObserver::
+                         RemoveObserverForSession,
+                     base::Unretained(frame_deletion_observer_.get()),
+                     iter->second->config.initial_context.render_process_id,
+                     iter->second->config.initial_context.render_frame_id,
+                     session_id));
+
   iter->second->ui.reset();
 
   base::ThreadTaskRunnerHandle::Get()->PostTask(