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This CL uses the finch trial group name to set WebKit settings for the android autoplay gesture override experiment.  Most of the code for the experiment itself is in https://codereview.chromium.org/1179223002/ .  The present CL will not build without it.

Summary of the experiment: Our goal is to decide if the requirement for a user gesture can be relaxed on android, to allow some media to autoplay without always requiring the user to interact with it manuall.  The finch experiment will let us try various combinations of heuristics for when to autoplay without a user gesture.  We record metrics based on user behavior, such as "manually started playback", "manually stopped playback", etc.

These options are composible, so we just use the trial group name to specify the options directly.  This lets us try different combinations after we get some initial data from the experiment.

BUG=487345, 402044

Review URL: https://codereview.chromium.org/1331203003

Cr-Commit-Position: refs/heads/master@{#348762}