[WebApk] Enable WebApks by default.

Maintains the finch flag so that we can disable if the server encounters
issues but removes about:flag and defaults to enabled.
BUG=629181

Change-Id: I2fafaee9cf6227a0683983790ea2bb20d90197a8
Reviewed-on: https://chromium-review.googlesource.com/620532
Reviewed-by: Alexei Svitkine (very slow) <asvitkine@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495601}
6 files changed