Change NTPSnippetsService to implement ContentSuggestionsProvider

The NTPSnippetsService currently delivers snippets to the UI via the
Android bridge. Change it to additionally serve as a
ContentSuggestionsProvider implementation and deliver
ContentSuggestions for the ARTICLES category. These ContentSuggestions
are currently only delivered to the ContentSuggestionsService and not
served to the UI, yet.

Adjust NTPSnippetsServiceTest, SnippetsBridge and SnippetsInternals
because of a few renamed functions.

Change NTPSnippetsServiceFactory to register the NTPSnippetsService
as a provder with the ContentSuggestionsService.

BUG=619560

Review-Url: https://codereview.chromium.org/2131943002
Cr-Commit-Position: refs/heads/master@{#405103}
10 files changed