Don't wait forever if there are no commands to execute.

BUG=5614887

This fixes a timing issue where we could calculate a delay of 0 (indicating
wait forever) when we have no pending commands to actually execute. In such
cases, we should just break out of the playback loop.

This also fixes a small issue with returning whether or not to redraw.

Change-Id: Id1e481679341773256b7287062c68925e2bc8f9e