[milo] build_source -> buildSource.

milo/buildsource/rawpresentation/internal/stream.proto

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 syntax = "proto3";
 
 package internal;
 
 import "github.com/luci/luci-go/common/proto/milo/annotations.proto";
 
 // Item is a full annotation protobuf state item. It is used to serialize and
 // deserialize Step and associated fetch data into memcache.
 message CachedStep {
   // Step is the root annotation step.
   milo.Step step = 1;
 
   // Finished is true if this is the last annotation protobuf in the stream.
   bool finished = 2;
 }