[Content suggestions] Add a feature to fetch favicons from a new server

Together with CL 2781583002, this CL prepares fetching publishers
favicons in c++, using the new API function of LargeIconService.

In follow-up CLs, I will:
 - implement the actual functionality in ContentSuggestionsService,
 - switch in SnippetArticleViewHolder between the old and the new code
   path, based on the feature introduced in this CL.

BUG=705572

Review-Url: https://codereview.chromium.org/2781603002
Cr-Commit-Position: refs/heads/master@{#460678}
8 files changed