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.

docshell/base/nsDocShell.cpp

@@ -900,6 +900,7 @@ nsDocShell::nsDocShell()
   , mUseRemoteTabs(false)
   , mDeviceSizeIsPageSize(false)
   , mWindowDraggingAllowed(false)
+  , mInFrameSwap(false)
   , mCanExecuteScripts(false)
   , mFiredUnloadEvent(false)
   , mEODForCurrentDocument(false)
@@ -14074,3 +14075,16 @@ nsDocShell::GetPaymentRequestId(nsAString& aPaymentRequestId)
   aPaymentRequestId = GetInheritedPaymentRequestId();
   return NS_OK;
 }
+
+bool
+nsDocShell::InFrameSwap()
+{
+  nsRefPtr<nsDocShell> shell = this;
+  do {
+    if (shell->mInFrameSwap) {
+      return true;
+    }
+    shell = shell->GetParentDocshell();
+  } while (shell);
+  return false;
+}