vp8 dec: Add flag to bring up threads.

Instead of creating a new decoder instance when restarting all threads
after they were shut down, re-create threads on the new flag.

BUG=webm:1577

Change-Id: I6272ecaa1b586afdaa5ed8d6eab80aff8f5eb673
(cherry picked from commit 7be8d2df6c1c224a96b75d8f2ed66df7cbd0bb48)
5 files changed