[M73 merge] Assistant: Fix media session binding multiple times

This patch fixes the bug that the Assistant media session could bind
multiple times.

TBR=xiaohuic@chromium.org

Bug: 926051
Test: manual
Change-Id: I9d11724257ae96f93719e2cc3852d107c4686b30
Reviewed-on: https://chromium-review.googlesource.com/c/1441796
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#627231}(cherry picked from commit 2e7b4eb3ef48460af6e33b1ab900da07478193bb)
Reviewed-on: https://chromium-review.googlesource.com/c/1448700
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#95}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
diff --git a/chromeos/services/assistant/media_session/assistant_media_session.cc b/chromeos/services/assistant/media_session/assistant_media_session.cc
index 9c895ba..2b290d2 100644
--- a/chromeos/services/assistant/media_session/assistant_media_session.cc
+++ b/chromeos/services/assistant/media_session/assistant_media_session.cc
@@ -59,6 +59,7 @@
 
   // Create a mojo interface pointer to our media session.
   media_session::mojom::MediaSessionPtr media_session;
+  binding_.Close();
   binding_.Bind(mojo::MakeRequest(&media_session));
   audio_focus_ptr_->RequestAudioFocus(
       mojo::MakeRequest(&request_client_ptr_), std::move(media_session),