Add tracing service and make the shell+sky_viewer+services talk to it

services/tracing/trace_data_sink.cc

+// Copyright 2014 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 "services/tracing/trace_data_sink.h"
+
+#include "base/files/file_util.h"
+
+namespace tracing {
+
+TraceDataSink::TraceDataSink(base::FilePath base_name) : empty_(true) {
+  file_ =
+      base::OpenFile(base_name.AddExtension(FILE_PATH_LITERAL(".trace")), "w+");
+  static const char start[] = "{\"traceEvents\":[";
+  fwrite(start, 1, strlen(start), file_);
+}
+
+TraceDataSink::~TraceDataSink() {
+  if (file_)
+    Flush();
+  DCHECK(!file_);
+}
+
+void TraceDataSink::AddChunk(const std::string& json) {
+  if (!empty_) {
+    fwrite(",", 1, 1, file_);
+  }
+
+  empty_ = false;
+
+  fwrite(json.data(), 1, json.length(), file_);
+}
+
+void TraceDataSink::Flush() {
+  static const char end[] = "]}";
+  fwrite(end, 1, strlen(end), file_);
+  base::CloseFile(file_);
+  file_ = nullptr;
+}
+
+}  // namespace tracing