GitLab

Adding a UMA to keep track of the number of times RequestMediaKeyAccess
is called with a particular key system, and the number of times the
request succeeds. The counts are only incremented once per renderer
frame.

BUG=351501
TEST=existing EME tests pass, verified UMA locally

Review URL: https://codereview.chromium.org/881853002

Cr-Commit-Position: refs/heads/master@{#313956}