Make chrome://flags control whether the WebAPK feature is on

This CL removes the need for special code in variations to allow users to
experiment with WebAPKs. Since "variations experiments" are time limited, this
is a good thing.

This CL removes the ability to disable the WebAPK feature through variations.
This is OK because very few users currently use WebAPKs. In order to use
WebAPKs users need to turn the feature on via chrome://flags Once we start
rolling out WebAPKs to all users we will need to add a variations parameter as
a kill switch

BUG=None

Review-Url: https://codereview.chromium.org/2544973002
Cr-Commit-Position: refs/heads/master@{#438425}
10 files changed