Refactor UKM interface for mojo-ification

components/metrics/proto/ukm/entry.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;
 
 // Next tag: 6
 message Entry {
   // Unique identifier for an Event. This is used to provide hierarchical
   // structure for Events.
   optional int32 id = 1;
 
   // Parent id, which is associated with an id for another Event. This is set
   // only if the Event has another Event which can be considered a parent.
   // This is to provide hierarchies for Events.
   optional int32 parent_id = 2;
 
   // The id of the Source this Event is associated with.
-  optional int32 source_id = 3;
+  optional int64 source_id = 3;
 
   // Type of the Event. This is a hash of the name (as a string).
   optional fixed64 event_hash = 4;
 
   // For each Event, we have a list of possible metrics included. It's possible
   // for a single metric name to be repeated. There is also no guarentee that
   // all metrics that are available for a given event will be provided for a
   // single Entry.
   message Metric {
     optional fixed64 metric_hash = 1;
     optional int64 value = 2;
   }
   repeated Metric metrics = 5;
 }