Improve the testing on MojoCdm by adding a MockCdm. This allows for
testing that:
- resolving the promise works
- rejecting the promise works
- closing the connection before a call works
- closing the connection while executing the call works.

BUG=671362
TEST=new tests pass

Review-Url: https://codereview.chromium.org/2592913002
Cr-Commit-Position: refs/heads/master@{#442104}