Bug 1191491 - Do not dispatch an audio-playback notification when swapping browsers; r=smaug

We send a pagehide event during swapping docshell frame loaders, and
before this patch we would not be able to differentiate this event with
the one that we send when navigating away from a page, so we would
incorrectly dispatch an audio-playback notification indicating that
audio playback has stopped.  This patch adds a flag to the root docshell
when the frame loader swapping is in progress and disables the above
behavior when that flag is set.

dom/html/HTMLMediaElement.cpp

@@ -103,6 +103,7 @@ static PRLogModuleInfo* gMediaElementEventsLog;
 
 #include "nsIPermissionManager.h"
 #include "nsContentTypeParser.h"
+#include "nsDocShell.h"
 
 #include "mozilla/EventStateManager.h"
 
@@ -4013,7 +4014,14 @@ void HTMLMediaElement::NotifyOwnerDocumentActivityChanged()
   if (pauseElement && mAudioChannelAgent) {
     // If the element is being paused since we are navigating away from the
     // document, notify the audio channel agent.
-    NotifyAudioChannelAgent(false);
+    // Be careful to ignore this event during a docshell frame swap.
+    auto docShell = static_cast<nsDocShell*>(OwnerDoc()->GetDocShell());
+    if (!docShell) {
+      return;
+    }
+    if (!docShell->InFrameSwap()) {
+      NotifyAudioChannelAgent(false);
+    }
   }
 }