Use AudioStreamMonitor to control power save blocking.

chrome/browser/media/audio_stream_monitor.cc

Index: chrome/browser/media/audio_stream_monitor.cc
diff --git a/chrome/browser/media/audio_stream_monitor.cc b/chrome/browser/media/audio_stream_monitor.cc
index 8633053d510684d1a69143590ec9d3047eb44194..4e531f3afc4c61ca46a664a7a42ac97171e5f455 100644
--- a/chrome/browser/media/audio_stream_monitor.cc
+++ b/chrome/browser/media/audio_stream_monitor.cc
@@ -7,6 +7,7 @@
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "content/public/browser/invalidate_type.h"
+#include "content/public/browser/power_save_blocker.h"
 #include "content/public/browser/web_contents.h"
 
 DEFINE_WEB_CONTENTS_USER_DATA_KEY(AudioStreamMonitor);
@@ -77,7 +78,15 @@ void AudioStreamMonitor::MaybeToggle() {
 
   if (!should_indicator_be_on) {
     off_timer_.Stop();
+    blocker_.reset();
   } else if (!off_timer_.IsRunning()) {
+#if !defined(OS_CHROMEOS) && !defined(OS_ANDROID)

[miu, 2014/08/25 19:34:52]

Hmm...I'm wondering whether this is the right place for creating/owning/etc the
PowerSaveBlocker, since the purpose of AMM is just to monitor things.

However, I'm not sure where would be a better place.  It would be iffy to put
this in the UI code that handles the INVALIDATE_TYPE_TAB events from
WebContents.  Another solution would be to roll your own WebContentsObserver
whose sole purpose is to call
AMM::FromWebContents(contents)->WasRecentlyAudible() each time navigation state
has changed.

[DaleCurtis, 2014/08/28 00:55:19]

I also wonder about this, as now the code lives across chrome and content; only
by their powers combined does it do the right thing. On the other hand it seems
unfortunate to reimplement this code. Could AudioStreamMonitor be moved to
content? Should this be integrated with WebContentsImpl some how?

[miu, 2014/08/28 04:11:58]

My opinion: Either ASM needs to move into content, or the video side of
PowerSaveBlocking needs to move into chrome.  Do we consider power save blocking
a browser-level feature, or one necessary for rendering/managing web content?

(Side note: One argument in favor of moving ASM into content is that other
browsers that use libcontent, such as Opera, would benefit from the intelligent
audio power save blocking.)

Obviously ASM's in chrome right now because it's only used for browser-level
features: the tab UI indicator and OOM killer.  Moving it into content would
require introducing a public content API.

OTOH, the steps for moving ASM into content wouldn't be too bad, but not exactly
trivial either:

1. Move the audio_stream_monitor.h/.cc files to content/browser/media, and place
the ASM class in the content namespace.  ASM has no dependencies that would make
this difficult.

2. Introduce a public content API that allows chrome code (e.g. tab_utils.cc) to
access ASM functionality.

3. Unlgue ASM from chrome/browser/media/media_capture_devices_dispatcher.cc and
just have AudioRendererHost start/stop it directly instead.  Currently, ARH
invokes MediaCaptureDevicesDispatcher through the MediaObserver interface.

[DaleCurtis, 2014/08/28 18:51:43]

I think we should, the ASM stuff is pretty awesome and letting dependent
browsers get access to it would be great.
This doesn't seem too bad.  I'll move forward with this assuming sky/jam are
onboard.

+    if (!blocker_) {
+      blocker_ = content::PowerSaveBlocker::Create(
+          content::PowerSaveBlocker::kPowerSaveBlockPreventDisplaySleep,

[miu, 2014/08/25 19:34:52]

You meant "AppSuspension" and not "DisplaySleep" right?

[DaleCurtis, 2014/08/28 00:55:19]

Acknowledged.

+          "Playing Audio");
+    }
+#endif
     off_timer_.Start(
         FROM_HERE,
         off_time - now,