SuggestionsService: implement SyncServiceObserver
so that its clients don't have to.

Also remove a bunch of null checks on SuggestionsService - SuggestionsServiceFactory::GetForProfile will never return null.

BUG=591973

Review URL: https://codereview.chromium.org/1770843002

Cr-Commit-Position: refs/heads/master@{#380603}
diff --git a/components/suggestions.gypi b/components/suggestions.gypi
index efffdef..dbbfe64d 100644
--- a/components/suggestions.gypi
+++ b/components/suggestions.gypi
@@ -19,6 +19,7 @@
         'components.gyp:data_use_measurement_core',
         'components.gyp:keyed_service_core',
         'components.gyp:pref_registry',
+        'components.gyp:sync_driver',
         'components.gyp:variations',
         'components.gyp:variations_net',
       ],
@@ -37,8 +38,6 @@
         'suggestions/suggestions_service.h',
         'suggestions/suggestions_store.cc',
         'suggestions/suggestions_store.h',
-        'suggestions/suggestions_utils.cc',
-        'suggestions/suggestions_utils.h',
       ],
       'variables': {
         'proto_in_dir': 'suggestions/proto',