Remove CDM_7 support

All CDMs released since M43 support the CDM_8 interface, so remove
support for CDM_7 as it is no longer used.

BUG=640437
TEST=browser_tests pass

Review-Url: https://codereview.chromium.org/2280433002
Cr-Commit-Position: refs/heads/master@{#414471}
7 files changed