Adds tracking for
- User actions:
  + ContentSuggestions.RemoteSuggestionsPreferenceOn
  + ContentSuggestions.RemoteSuggestionsPreferenceOff
  + ContentSuggestions.NotificationsPreferenceOn
  + ContentSuggestions.NotificationsPreferenceOff
- Histograms:
  + NewTabPage.ContentSuggestions.Preferences.RemoteSuggestions (Boolean)
  + NewTabPage.ContentSuggestions.Notifications.Actions (added one enum value)

BUG=706384

Review-Url: https://codereview.chromium.org/2790183002
Cr-Commit-Position: refs/heads/master@{#463466}