Add UMA to study the video types for mobile

Before we work on transfering from MediaPlayer to MediaCodec, we want to get some statistics on the current mobile video types to see the impacts.
We want to know how many video are HLS(which will still use MediaPlayer) and how many videos are using MediaSource(which already uses MediaCodec)

BUG=401795

Review URL: https://codereview.chromium.org/505233002

Cr-Commit-Position: refs/heads/master@{#292756}
3 files changed