Remove theoretical race condition between Stop() and Initialize().
Remove theoretical race condition between Stop() and Initialize().

Initialize() could acquire lock_ and launch a new thread_ while Stop() had
un-lock_'d and was joining the old thread.  If that happened then Stop() would
silently leak the new thread when it re-acquired lock_ and set thread_ to NULL.

This also removes a cheap but unnecessary lock/unlock pair and clarifies the
function a bit.

Tested: built & ran media_unittests.

Patch by fischman@chromium.org:
http://codereview.chromium.org/6531012/

TEST=none
BUG=none

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@75212 0039d316-1c4b-4281-b951-d872f2087c98

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