Add DecryptingDemuxerStream::Stop().

During the teardown process, media pipeline will be waiting on the
render main thread. If a Decryptor depends on the render main thread
(e.g. PpapiDecryptor), the pending DecryptCB will not be satisfied.

This CL adds a DecryptingDemuxerStream::Stop() method so that the
stopping process doesn't wait for any pending operation.

BUG=343748
TEST=Add unittests to cover this case.
R=ddorwin@chromium.org

Review URL: https://codereview.chromium.org/165733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251277 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251277