Previously when using pepper CDM, Clear Key CDM (adapter) was registered
as a pepper plugin, using --register-pepper-plugins. As we are moving to
use mojo CDM, this will not work. This CL introduces a new command line
switch --clear-key-cdm-path-for-testing to specify the path to the CDM.
Since we are not using pepper, we don't need to specify anything related
to the adapter, the display name, or the mime type.

Note that key system support query is still using pepper plugin info. So
even for tests using mojo CDM, we still need to RegisterPepperCdm() for
now. We will fix this in a future CL.

BUG=764114
TEST=Existing tests work.

Change-Id: I246446eae690df275378afa32ccb7bef4b055dc2
Reviewed-on: https://chromium-review.googlesource.com/662280


Reviewed-by: John Rummell <jrummell@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501848}