Use base::PowerObserver to check for missed heartbeats

Use base::PowerObserver to check for missed heartbeats

base::PowerObserver provides a platform-agnostic way to be notified
about system suspend and resume events.  Use it to check for missed
heartbeats instead of polling.  Unfortunately we still need to poll on
linux because there is no common system-wide notification about suspend
and resume events.

BUG=421725
Committed: https://crrev.com/7f8789c70bb9049c9357f512e92f8b44c061e892
Cr-Commit-Position: refs/heads/master@{#319730}

[Chirantan Ekbote, 2015-03-07 02:16:23]

chirantan@chromium.org changed reviewers:
+ zea@chromium.org

[Chirantan Ekbote, 2015-03-07 02:16:23]

Hi Nicolas, please take a look.

[Nicolas Zea, 2015-03-09 18:42:52]

LGTM with a question. Have we had this power monitor all along?

https://codereview.chromium.org/989803002/diff/1/google_apis/gcm/engine/heart...
File google_apis/gcm/engine/heartbeat_manager.cc (right):

https://codereview.chromium.org/989803002/diff/1/google_apis/gcm/engine/heart...
google_apis/gcm/engine/heartbeat_manager.cc:116: CheckForMissedHeartbeat();
On Linux, will this just never be called?

[Chirantan Ekbote, 2015-03-09 19:23:59]

On 2015/03/09 18:42:52, Nicolas Zea wrote:
> LGTM with a question. Have we had this power monitor all along?
> 

I don't know exactly when it was created but it looks like it's been there for a
while.  The git blame for power_monitor shows that it was created in 2013 when
it was split out from system_monitor, and the git blame for system_monitor goes
back to 2011.

https://codereview.chromium.org/989803002/diff/1/google_apis/gcm/engine/heart...
File google_apis/gcm/engine/heartbeat_manager.cc (right):

https://codereview.chromium.org/989803002/diff/1/google_apis/gcm/engine/heart...
google_apis/gcm/engine/heartbeat_manager.cc:116: CheckForMissedHeartbeat();
On 2015/03/09 18:42:51, Nicolas Zea wrote:
> On Linux, will this just never be called?

Yup, that's correct.

[Chirantan Ekbote, 2015-03-09 19:24:15]

The CQ bit was checked by chirantan@chromium.org

[commit-bot: I haz the power, 2015-03-09 19:25:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/989803002/1

[commit-bot: I haz the power, 2015-03-09 20:49:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-09 20:50:06]

Patchset 1 (id:??) landed as
https://crrev.com/7f8789c70bb9049c9357f512e92f8b44c061e892
Cr-Commit-Position: refs/heads/master@{#319730}