Revert 138216 - [Sync] Turn notifier::PushClient into an interface

Revert 138216 - [Sync] Turn notifier::PushClient into an interface

Split the previous implementation into two pieces:
XmppPushClient and NonBlockingPushClient.

Add FakePushClient and FakePushClientObserver.

Remove use of ThreadSafeObserverList.

Add PushClient::CreateDefault() function, which creates a
NonBlockingPushClient for an XmppPushClient.

Dep-inject PushClient into P2PNotifier.

Add some helper functions to notification_defines.{h,cc}.

BUG=76764
TEST=

Review URL: https://chromiumcodereview.appspot.com/10413014

TBR=akalin@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=138221