[NTP Snippets] Cache images in a LevelDB

components/ntp_snippets/proto/ntp_snippets.proto

 // Copyright 2016 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 ntp_snippets;
 
 message SnippetSourceProto {
   optional string url = 1;
   optional string publisher_name = 2;
   optional string amp_url = 3;
 }
 
 message SnippetProto {
   optional string id = 1;
   optional string title = 2;
   optional string snippet = 3;
   optional string salient_image_url = 4;
   optional int64 publish_date = 5;
   optional int64 expiry_date = 6;
   optional float score = 7;
   repeated SnippetSourceProto sources = 8;
   optional bool discarded = 9;
 }
+
+message SnippetImageProto {
+  optional bytes data = 1;
+}