Fix race in Pipeline where video_decoder_ was accessed from multiple threads. I moved the code that clears the variable to StopTask(). The problem was that all access to this variable except for inside Stop(), happens on the Pipeline thread, but was not protected by a lock. So, Stop() could be called and it could cause random crashes on the pipeline thread such as in the case I mention in the below bug.
Fix race in Pipeline where video_decoder_ was accessed from multiple threads.
I moved the code that clears the variable to StopTask(). The problem was that
all access to this variable except for inside Stop(), happens on the Pipeline
thread, but was not protected by a lock. So, Stop() could be called and it
could cause random crashes on the pipeline thread such as in the case I
mention in the below bug.

BUG=115299
TEST=follow repro steps in the bug report.


Review URL: http://codereview.chromium.org/9536009

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@124208 0039d316-1c4b-4281-b951-d872f2087c98

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