Rollup of bug 819000: Preload about:blank. r=cjones,jlebar a=cjones

Includes the following patches

Bug 819000: Bail out of PresShell::Paint if not active. r=roc
Bug 819000: Make sure preloaded docshells are reset to active. r=jlebar
Bug 819000: Preload about:blank. r=cjones a=cjones
Bug 830648 - AudioChannelAgent::StartPlaying() should check if already registered to AudioChannelService., r=mchen, a=blocking-b2g
Bug 829561 - AudioChannelService needs some unit test, r=mchen, a=blocking-b2g