Restore preroll state when decoder job created

Adds resumption of MediaDecoderJob preroll when MediaSourcePlayer
re-creates the job(s), and adds related MediaSourcePlayerTests.
Includes test hooks to verify job's prerolling status, also to be used by eventual fix for bug 309184.

R=qinmin@chromium.org,acolwell@chromium.org
BUG=309122, 309184
TEST=All MediaSourcePlayerTests pass on Android with MediaCodecBridge available.

Review URL: https://codereview.chromium.org/33543004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230600 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230600