Add TrackableCallback in ContentDecryptorDelegate.

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
Ignore offset parameter in encode callback. It is ignored/deprecated by the caller, and bitmap.pixelRefOffset() is also deprecated

BUG=

Review URL: https://codereview.chromium.org/116693005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243461 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243461