Register only one plugin in EncryptedMediaTest.

- Only register ClearKeyCdm when we test external clear key. Only register
Widevine CDM when we test Widevine.
- Add WVEncryptedMediaTest so that we can test non-parameterized tests.

BUG=307745
TEST=none
R=ddorwin@chromium.org

Review URL: https://codereview.chromium.org/27441004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229492 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229492