Add shell::Tracer object and wire up to --trace-startup on Android

shell/tracer.h

Index: shell/tracer.h
diff --git a/shell/tracer.h b/shell/tracer.h
new file mode 100644
index 0000000000000000000000000000000000000000..4af166bd2e8a3a419996e429e7b25b7b820ed3d7
--- /dev/null
+++ b/shell/tracer.h
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+// Copyright 2015 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.
+
+#ifndef SHELL_TRACER_H_
+#define SHELL_TRACER_H_
+
+#include <stdio.h>
+
+#include <string>
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted_memory.h"
+
+namespace shell {
+
+// Tracer collects tracing data from base/trace_event and from externally
+// configured sources, aggregates it into a single stream, and writes it out to
+// a file. It should be constructed very early in a process' lifetime before any
+// initialization that may be interesting to trace has occured and be shut down
+// as late as possible to capture as much initialization/shutdown code as
+// possible.
+class Tracer {
+ public:
+  Tracer();
+  ~Tracer();
+
+  // Starts tracing the current process with the given set of categories.
+  void Start(const std::string& categories);
+
+  // Stops tracing and flushes all collected trace data to the given filename.
+  // Blocks until the file write is complete. May be called after the message
+  // loop is shut down.
+  void StopAndFlushToFile(const std::string& filename);
+
+ private:
+  void StopTracingAndFlushToDisk(const std::string& filename);
+
+  // Called from the flush thread. When all data is collected this runs
+  // |done_callback| on the flush thread.
+  void EndTraceAndFlush(const std::string& filename,
+                        const base::Closure& done_callback);
+
+  // Called from the flush thread.
+  void WriteTraceDataCollected(
+      const base::Closure& done_callback,
+      const scoped_refptr<base::RefCountedString>& events_str,
+      bool has_more_events);
+
+  // Whether we're currently tracing. Main thread only.
+  bool tracing_;
+
+  // Whether we've written the first chunk. Flush thread only.
+  bool first_chunk_written_;
+
+  // Trace file, if open. Flush thread only.
+  FILE* trace_file_;
+
+  DISALLOW_COPY_AND_ASSIGN(Tracer);
+};
+
+}  // namespace shell
+
+#endif  // SHELL_TRACER_H_