GitLab

There are still some TODOS (most notably incognito is not supported),
but this can largely be used outside of that.

Embedder usage is as follows:
. embedders interested in favicons call Tab::CreateFaviconFetcher() supplying
  a delegate. At this time embedders don't get the ability to specify a
  size (I wouldn't be surprised if we need this later). The actual size
  requested depends upon the platform.
. The delegate is notified any time the favicon of the current navigation
  changes (not including when a new navigation starts).
. The current favicon can be requested from the FaviconFetcher returned from
  CreateFaviconFetcher.

Implementation wise the following pieces are used:
. FaviconServiceImpl: implementation of favicon::CoreFaviconService that
  is used on the main thread to interact with WebLayerFaviconBackend.
. WebLayerFaviconBackend: owns the database. All methods largely delegate to
  the database (same database used in chrome).
. FaviconTabHelper: responsible for creating favicon::ContentFaviconDriver
  as necessary. ContentFaviconDriver is ultimately responsible for getting
  the favicon for the current navigation. ContentFaviconDriver is only
  created when necessary (meaning CreateFaviconFetcher() was called) as it
  actively downloads favicons.
. FaviconFetcherImpl: created via Tab::CreateFaviconFetcher(), uses
  FaviconTabHelper to handle notifying delegate and maintaining favicon.

BUG=1076463
TEST=covered by weblayer_browsertests

Change-Id: I74bce59a6a46c38000b37b028a2d3e7797680e33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337275
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795110}