[dom_distiller] Add support for incremental viewer.

components/dom_distiller/core/proto/distilled_page.proto

 // Copyright 2013 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.
 //
 // Distilled page content.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
@@ skipping 10 matching lines @@
     // Handle to the image. This could be the URL where the image was found.
     // The handle should be referenced verbatim in the HTML.
     required string name = 1;
 
     // The image data
     required bytes data = 2;
   }
 
   // The images referenced in the HTML.
   repeated Image image = 4;
-}
+
+  // Title for the current page.
+  optional string title = 5;
+}