[Media Session] Fix crash on abandon

Fix a race condition when notifying observers
that a session was abandoned.

BUG=971827

Change-Id: Ib0ec8188e7936cee2cd6676709dde408bc3ab3d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648951Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667256}

services/media_session/audio_focus_manager.cc

@@ -111,8 +111,10 @@ void AudioFocusManager::AbandonAudioFocusInternal(RequestId id) {
   MaybeUpdateActiveSession();
 
   // Notify observers that we lost audio focus.
-  observers_.ForAllPtrs([&row](mojom::AudioFocusObserver* observer) {
-    observer->OnFocusLost(row->ToAudioFocusRequestState());
+  mojom::AudioFocusRequestStatePtr session_state =
+      row->ToAudioFocusRequestState();
+  observers_.ForAllPtrs([&session_state](mojom::AudioFocusObserver* observer) {
+    observer->OnFocusLost(session_state.Clone());
   });
 
   if (!was_top_most_session || audio_focus_stack_.empty())