media: Add use counter for EME policy check

Report a count when MediaKeys::getStatusForPolicy() is called so that we
keep track of the usage of this API.

BUG=709348

Change-Id: Iee2080d22406019fe76c66e9463c4f253a8dbd5e
Reviewed-on: https://chromium-review.googlesource.com/827633
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524831}
3 files changed