Add missing IWYU message_loop.h includes.

base/trace_event/blame_context_unittest.cc

 // 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.
 
 #include "base/trace_event/blame_context.h"
 
 #include "base/json/json_writer.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted_memory.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/test/trace_event_analyzer.h"
 #include "base/trace_event/trace_buffer.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 namespace trace_event {
 namespace {
 
@@ skipping 193 matching lines @@
       "}";
 
   std::string snapshot_json;
   JSONWriter::Write(*events[2]->GetKnownArgAsValue("snapshot"), &snapshot_json);
   EXPECT_EQ(kExpectedSnapshotJson, snapshot_json);
 }
 
 }  // namepace
 }  // namespace trace_event
 }  // namespace base