WebApp: Extract web_applications source sets in GN.

This CL reflects the existing dependencies and dependency
cycles we have.

Also, we want to establish the following folders in web_applications/:
1) components/
Any independent components which are shared between extensions/
and web_applications/

2) extensions/
A code which is thematically part of chrome/browser/extensions/
E.g. an upcoming BookmarkAppInstaller will be used by extensions.

3) bookmark_apps/
Any extension-based high-level helpers which are used by chrome/browser/

Note that web_applications/policy content will be moved to
web_applications/bookmark_apps/policy/

Bug: 860581
Change-Id: Ib89f0e327759cfab050caceae5818c8e72f40f08
Reviewed-on: https://chromium-review.googlesource.com/1128665
Commit-Queue: Alexey Baskakov <[email protected]>
Reviewed-by: Trent Apted <[email protected]>
Reviewed-by: Devlin <[email protected]>
Reviewed-by: Dominick Ng <[email protected]>
Cr-Commit-Position: refs/heads/master@{#574448}
9 files changed