Fixed the audio backgrounding bug

content/browser/renderer_host/render_process_host_impl.cc

Index: content/browser/renderer_host/render_process_host_impl.cc
diff --git a/content/browser/renderer_host/render_process_host_impl.cc b/content/browser/renderer_host/render_process_host_impl.cc
index 3471e3c087df2e43716a1bd742defd22d7326c79..b3b1c5d07605dca7d81855c0636bc8ffb2fdda8d 100644
--- a/content/browser/renderer_host/render_process_host_impl.cc
+++ b/content/browser/renderer_host/render_process_host_impl.cc
@@ -1092,6 +1092,28 @@ int RenderProcessHostImpl::VisibleWidgetCount() const {
   return visible_widgets_;
 }
 
+void RenderProcessHostImpl::AudioStopped() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  // If the backgrounding was skipped because audio was playing, background the
+  // tab now.
+  if (backgrounded_)
+    SetBackgrounded(true);
+}
+
+void RenderProcessHostImpl::AudioStarted() {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+
+  // If audio starts playing from a backgrounded tab, un-background it.
+  if (backgrounded_) {
+    SetBackgrounded(false);
+
+    // Since it's not a visible tab, it needs to be marked for backgrounding
+    // again so that it is re-backgrounded if/when the audio stops.
+    SetBackgrounded(true);

[ncarter (slow), 2015/07/06 21:53:57]

I think we should fix things up so that this hack isn't necessary. Here's a
sketch of what I'm thinking: https://codereview.chromium.org/1214393004 

[gab, 2015/07/08 18:52:14]

Proposed skeleton SGTM, the AudioRendererHost could then either call
UpdateProcessPriority() itself or, for potentially better abstraction, could
take it as a Closure to be called when the audio-playing state changes (any
preference on your end?).

Dropped a few comments on https://codereview.chromium.org/1214393004 just in
case Seb intended to copy it as-is.

[sebsg, 2015/08/02 16:44:23]

Done.

[sebsg, 2015/08/02 16:44:23]

Done.

+  }
+}
+
 bool RenderProcessHostImpl::IsForGuestsOnly() const {
   return is_for_guests_only_;
 }
@@ -2237,6 +2259,7 @@ void RenderProcessHostImpl::SuddenTerminationChanged(bool enabled) {
 }
 
 void RenderProcessHostImpl::SetBackgrounded(bool backgrounded) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
   TRACE_EVENT1("renderer_host", "RenderProcessHostImpl::SetBackgrounded",
                "backgrounded", backgrounded);
   // Note: we always set the backgrounded_ value.  If the process is NULL