Add support for multipage distillation.

components/dom_distiller/core/proto/distilled_article.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.
+//
+// Distilled article content.
+
+syntax = "proto2";
+
+option optimize_for = LITE_RUNTIME;
+
+package dom_distiller;
+
+import "distilled_page.proto";
+
+message DistilledArticleProto {
+  // A title for the article.
+  optional string title = 1;
+
+  // The distilled pages in the article.
+  repeated DistilledPageProto pages = 2;
+}