WebView: Ignore permanent-consistency experiments

Android WebView currently treats permanent-consistency experiments
(which are intended to retain their group assignments across runs) as
session-consistency (which are intended to be randomly assigned on each
run). It's safer to not apply permanent-consistency experiments to
WebView at all, to avoid mistakes.

BUG=866722

Change-Id: I9d501437718d934c2a59b95335c787302ce758e7
Reviewed-on: https://chromium-review.googlesource.com/1148991
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578070}
8 files changed