[AudioStreamMonitor] Adds API to collect frame-level audibility.

content/browser/frame_host/render_frame_host_impl.cc

Index: content/browser/frame_host/render_frame_host_impl.cc
diff --git a/content/browser/frame_host/render_frame_host_impl.cc b/content/browser/frame_host/render_frame_host_impl.cc
index 1be87def8bdc84d7b92ddf9a2a25ae6c47946760..5fc45cff6bde2780fa0151fc38ab1235cb73b3dd 100644
--- a/content/browser/frame_host/render_frame_host_impl.cc
+++ b/content/browser/frame_host/render_frame_host_impl.cc
@@ -437,6 +437,7 @@ RenderFrameHostImpl::RenderFrameHostImpl(SiteInstance* site_instance,
       pending_web_ui_type_(WebUI::kNoWebUI),
       should_reuse_web_ui_(false),
       has_selection_(false),
+      is_audible_(false),
       last_navigation_previews_state_(PREVIEWS_UNSPECIFIED),
       frame_host_interface_broker_binding_(this),
       frame_host_associated_binding_(this),
@@ -995,6 +996,9 @@ void RenderFrameHostImpl::RenderProcessGone(SiteInstanceImpl* site_instance) {
   // Any future UpdateState or UpdateTitle messages from this or a recreated
   // process should be ignored until the next commit.
   set_nav_entry_id(0);
+
+  if (is_audible_)
+    GetProcess()->OnAudioStreamRemoved();
 }
 
 void RenderFrameHostImpl::ReportContentSecurityPolicyViolation(
@@ -1182,6 +1186,16 @@ void RenderFrameHostImpl::Init() {
   }
 }
 
+void RenderFrameHostImpl::OnAudioStateChanged(bool is_audible) {
+  if (is_audible_ == is_audible)
+    return;
+  if (is_audible)
+    GetProcess()->OnAudioStreamAdded();

[miu, 2017/06/26 22:57:59]

The OnAudioStreamAdded/Removed() methods need to be renamed. I'm not sure what
their original purpose was: Do they have any other callers besides this code?
Since they are now being called to report changes in audibility, and not whether
audio streams have actually been created or destroyed, I'd suggest something
like:

  OnRenderFrameBecameAudible()
  OnRenderFrameBecameSilent()

[lpy, 2017/06/28 02:14:29]

I will leave it to separate patches.

+  else
+    GetProcess()->OnAudioStreamRemoved();
+  is_audible_ = is_audible;
+}
+
 void RenderFrameHostImpl::OnDidAddMessageToConsole(
     int32_t level,
     const base::string16& message,