GitLab

- Creates DiscoverFeedService, a BrowserState KeyedService
- Creates DiscoverFeedServiceFactory to create and own
DiscoverFeedServices
- Initializes DiscoverFeedService in main controller.
- Updates DiscoverFeedProvider interface with methods to start the Feed
and update when the Identity changes.

Bug: 1085419
Change-Id: Ie39653f6263915f9ef69b68d5c1e2182d15c7ab1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2344503Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798272}