GitLab

Add OnSuggestionInvalidated methods to the ContentSuggestionsProvider,
the ContentSuggestionsService and its observer and to the SnippetsBridge.

Remove suggestions from the UI immediately when they're invalidated.

Invalidate offline page suggestions when the referenced offline page is
deleted, and also clear it from the dismissed_ids list.

Add new unit tests for the UI layer and the service layer.

BUG=628198

Review-Url: https://codereview.chromium.org/2244793002
Cr-Commit-Position: refs/heads/master@{#412813}