Revert 170660

Revert 170660

This change causes shutdown crashes when chrome menu is used to exit.

> Keep browser process alive while there are platform apps with background pages running. This does not happen on Android or ChromeOS builds: Android does not support browser process keep alive, and ChromeOS does not need or like it.
> 
> This change prevents platform apps getting killed unceremoniously while they have background pages active.
> 
> This also delays the process being shutdown after closing the last platform app window, as background pages are kept alive for 15 seconds after their last activity is completed.
> 
> This change re-lands r165772 which caused problems on Google branded ChromeOS. This version does not add keep alives for ChromeOS and was tested on the ChromeOS try bots.
> 
> TBR=davemoore@chromium.org for the ash launcher test change.
> 
> BUG=155457
> TEST=Make sure platform apps can be used from the launcher without chrome windows open; make sure Chrome shuts down properly in all cases.
> 
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=165772
> 
> Review URL: https://chromiumcodereview.appspot.com/11117011

TBR=benwells@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=170891