Android Media Notification: stop the MediaSession when dismissed.

The current behaviour is to simply dismiss the notification but the
WebContents and content::MediaSession will consider the session still
running while the user's intent was to actually stop it. As a result,
the WebContents will still receive notification about the session
changes, for example, when Chrome will lose/re-gain audio focus. These
notifications will re-introduce the Notification (in a paused state).

BUG=515129

Review URL: https://codereview.chromium.org/1294863002

Cr-Commit-Position: refs/heads/master@{#343688}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 1b066ba..fb770ca 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3407,6 +3407,10 @@
   MediaSession::Get(this)->Suspend();
 }
 
+void WebContentsImpl::StopMediaSession() {
+  MediaSession::Get(this)->Stop();
+}
+
 #endif  // defined(OS_ANDROID)
 
 void WebContentsImpl::OnFirstVisuallyNonEmptyPaint() {