GitLab

We want to erase web_app_extension_helpers.h in favor of
web_app_helpers.h.

A plan:
1) Make WebAppProvider a single entry point for all external web_appplications/
clients.
2) Introduce enable_web_app_off_extensions command line switch.
3) Create new App Sync Manager component to utilize upcoming
Unified Sync and Storage (USS) system.
4) Be able to switch between old extension-based bookmark_apps/ and
new off-extensions bookmark_apps2/ implementations inside the WebAppProvider.

TBR=sky@chromium.org

Bug: 860581
Change-Id: Iab19d936fbcf39c011fc1eff1e29125b5ddb5e16
Reviewed-on: https://chromium-review.googlesource.com/1159925
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580074}