[NTP Snippets] Persist snippets in a LevelDB instead of prefs

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;
+}