[Suggestions Service] Add support for expiring the SuggestionsStore

components/suggestions/proto/suggestions.proto

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package suggestions;
 
@@ skipping 24 matching lines @@
   optional string title = 2;
 
   // The URL of the favicon associated with this page.
   optional string favicon_url = 3;
 
   // The URL of the thumbnail associated with this page.
   optional string thumbnail = 4;
 
   // The provider(s) responsible for this suggestion.
   repeated ProviderId providers = 5;
+
+  // The timestamp (usec) at which this suggestion ceases to be valid.
+  optional int64 expiry_ts = 7;
 }
 
 // A list of URLs that should be filtered from the SuggestionsProfile.
 //
 // Next tag: 2
 message SuggestionsBlacklist {
   // URLs that make up the blacklist.
   repeated string urls = 1;
 }
 
 // ThumbnailData contains the data to represent a website thumbnail.
 //
 // Next tag: 3
 message ThumbnailData {
   // The URL of the website represented by this Thumbnail.
   optional string url = 1;
 
   // Bitmap bytes, encoded as JPEG.
   optional bytes data = 2;
 }