Move ClearKeyCdm files to media/cdm/ppapi/external_clear_key/.

TBR=ddorwin@chromium.org

Review URL: https://codereview.chromium.org/65553004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242550 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242550