cc: Unify BeginRetroFrames and non-RetroFrame paths in scheduler. [WIP]

cc: Unify BeginRetroFrames and non-RetroFrame paths in scheduler. [WIP]

This patch unifies the path to running BeginImplFrame no matter when it comes
in.

The theory works as follows;
 * Put BeginFrameArgs into a FIFO.
 * Scheduler processing of FIFO (which could be now).

FIFO processing works as follows;
 - While **more** than one args in the FIFO
  * Discard first args if they have expired

 - Now we either have;
  * Only on args left (which could be expired).
  * An unexpired args.

 - We now process them like we use too.


The idea behind the FIFO processing is;
 * We never only want to discard the last BeginFrameArgs we have because we
   don't have any mechanism which stops us discarding all of them. Plus, we
   don't really know when the next one will turn up either.

 * If we have multiple args, then its safe to discard the expired ones.

BUG=

[mithro-old, 2014-12-11 19:17:50]

mithro@mithis.com changed reviewers:
+ brianderson@chromium.org, sunnyps@chromium.org

[mithro-old, 2014-12-11 19:17:51]

At Brian's request, here is the other patch which begat our discussion about
when the correct time to run a BeginMainFrame discussion came from.

Tim