Add CWVFlags for configuring feature flags during runtime.

This will be used to point wallet and sync features to use the sandbox
servers to help with QA testing. //ios/web_view does not provide an
about:flags page but will rely on a public API that is only accessible
by developers.

Change-Id: I57fcd1e45ce4057468a1ab60de8f915d7ce1e937
Reviewed-on: https://chromium-review.googlesource.com/c/1404059
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Commit-Queue: John Wu <jzw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623823}
8 files changed