Add logs for EME API calls.

BUG=224786
TEST=Use --webcore-log-channels=Media in command line to enable these logs.

Review URL: https://codereview.chromium.org/145213002

git-svn-id: svn://svn.chromium.org/blink/trunk@165674 bbb929c8-8fbe-4397-9dbb-9b2b20218538