Encrypted Media: EME IPC naming cleanups.

- media_keys_id -> cdm_id
- InitializeCDM -> InitializeCdm
- CdmHostMsg_CreateSession_Type -> CdmHostMsg_CreateSession_ContentType

TBR=dcheng@chromium.org
BUG=315312

Review URL: https://codereview.chromium.org/177143003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254122 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254122