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The TrackableCallback manages the callback and the request ID so that we don't
need to manage them separately.

Also in this CL:
- Using media::Decryptor in .cc file.
- Renaming: pending_foo_cb_ -> foo_cb_

Review URL: https://codereview.chromium.org/105743010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243460 0039d316-1c4b-4281-b951-d872f2087c98