Add extract_text_only option

proto/dom_distiller.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";
 
 package dom_distiller.proto;
 option optimize_for = LITE_RUNTIME;
 option java_package = "com.dom_distiller.proto";
 option java_outer_classname = "DomDistillerProtos";
 
 message DistilledContent {
   optional string html = 1;
 }
 
 message PaginationInfo {
   optional string next_page = 1;
   optional string prev_page = 2;
   optional string canonical_page = 3;
 }
 
 message DomDistillerResult {
   optional string title = 1;
   optional DistilledContent distilled_content = 2;
   optional PaginationInfo pagination_info = 3;
   repeated string image_urls = 4;
 }
 
 message DomDistillerOptions {
+  // Whether to extract only the text (or to include the containing html).
+  optional bool extract_text_only = 1;
 }