Web Contents: expose the candidate favicon URLs in WebContents.

This will avoid WebContentsObserver to listen to the observer and
footgun themselves thinking that they will be aware of everything while
they should only expect updates.

WebContentsObserver documentation has been updated accordingly.

ContentFaviconDriver no longer exposes this information and the only
user is now using WebContents directly.

Bug: 1030540
Change-Id: I91ab4c193ef1ccbd3e4568250631029cb711f61c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1959334
Reviewed-by: Mikel Astiz <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Reviewed-by: Alexey Baskakov <[email protected]>
Commit-Queue: Mounir Lamouri <[email protected]>
Cr-Commit-Position: refs/heads/master@{#727166}
8 files changed