Add decrypt-only external clear key browser tests.

These tests cover the code path where a CDM is used in decrypt-only mode.

BUG=321420
TEST=none

Review URL: https://codereview.chromium.org/79903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236818 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236818