[Media] Add a finch trial param for background MediaSource videos

Added a new finch trial parameter to set the max average keyframe
distance that allows disabling of the video track or pausing
MediaSource videos when in the background.
WMPI now uses the right parameter according to the |load_type_|.
Parametrized unit tests test for both load types.

BUG=663999
TEST=unittests+manual

Review-Url: https://codereview.chromium.org/2818823002
Cr-Commit-Position: refs/heads/master@{#464619}
6 files changed