Bring up of the enhanced bookmarks cluster service.

components/enhanced_bookmarks/proto/cluster.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 ClusterRequest {
+  // Optional list of docs we want to cluster--a subset of the user's available
+  // docs.
+  repeated string docs = 1;
+
+  // When docs is empty, used to determine which clips are clustered.  If set to
+  // true, cluster all the user's clips.  Otherwise (default) cluster all of the
+  // user's uncategorized clips.
+  optional bool cluster_all = 3 [default = false];
+
+  extensions 2;
+}
+
+message ClusterResponse {
+  message Cluster {
+    repeated string docs = 1;
+    // May be empty or unset if no reasonable title could be found.
+    optional string title = 2;
+  }
+
+  // Each of ClusterRequest.docs will exist in exactly one cluster. Some
+  // clusters may be singletons, so
+  //   ClusterResponse.clusters_size() <= ClusterRequest.docs_size().
+  repeated Cluster clusters = 1;
+}