Basic UkmService implementation

components/metrics/proto/ukm/source.proto

+// Copyright 2017 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 ukm;
+
+option optimize_for = LITE_RUNTIME;
+
+// Source contains data related to a top-level navigation.
+// Next tag: 5
+message Source {
+  // A identifier for the source. This should be unique even across sessions.
+  optional int32 id = 1;
+
+  // The url of the source, as recorded in history.  If this URL does not
+  // appear in FirstTimes, it should be stripped before recording to logs.

[battre, 2017/01/12 08:12:55]

If the URL has not been discovered by Google's crawler, it must be stripped to
the security origin before recording to logs. If the security origin has not
been discovered by the crawler, the security origin must not be logged.

[Steven Holte, 2017/01/13 23:15:26]

Done.

+  optional string url = 2;
+
+  // Timestamp of navigation to this Source, as seen by the client. Time of
+  // events related to this Source will generally be relative to this timestamp.
+  optional int64 navigation_time_msec = 3;
+
+  // Time between navigation and the first meaningful paint, in milliseconds.
+  optional int64 first_meaningful_paint_msec = 4;
+}