Add method for removing bookmark image data.

components/enhanced_bookmarks/proto/metadata.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 image.collections;
 
 message ImageData {
   // Encrypted 64-bit image doc id, if it has been crawled,
   // e.g. "kY7_4LKgNqDrbM:"
   optional string doc_id = 1;
 
   message ImageInfo {
     // The (normalized) URL this image can be found at.
     optional string url = 1;
 
     // The dimensions in pixels.
     optional int32 width = 2;
     optional int32 height = 3;
   }
 
   // Information about the original collected image.
   optional ImageInfo original_info = 2;
 
   // Information about the server hosted thumbnail.
   optional ImageInfo thumbnail_info = 3;
+
+  // The expiration timestamp of the served thumbnail, in microseconds since
+  // epoch. The thumbnail is only guaranteed until this time, afterwards the
+  // URL may be broken.
+  // If expiration_timestamp is not present, then whoever set the thumbnail_info
+  // should guarantee that the thumbnail will not expire.
+  optional int64 expiration_timestamp = 5;
+
+  // Represents an explicit user action to remove an image. This will prevent
+  // any additional backfilling once this is set.
+  optional bool user_removed_image = 6;
 }
 
 message PageData {
   // The title of the web page.
   optional string title = 1;
 
   // A snippet of text from the web page, either computed by us or chosen by
   // the user.
   optional string snippet = 2;
 
   // The (normalized) URL of the web page.
   optional string url = 3;
 
   // The /url redirect signed URL for the web page.  This could be appended to
   // "www.google.com" to create a URL redirect.
   optional string signed_url = 5;
 
   // The doc id of the page, if in the index. Uses the same encrypted docid
   // format as ImageData.
   optional string doc_id = 4;
 }