Teach the mojo_shell --trace-startup flag to gather data from services

mojo/common/trace_controller_impl.h

 // 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 MOJO_COMMON_TRACING_CONTROLLER_IMPL_H_
 #define MOJO_COMMON_TRACING_CONTROLLER_IMPL_H_
 
 #include "base/memory/ref_counted_memory.h"
 #include "mojo/public/cpp/bindings/interface_request.h"
 #include "mojo/public/cpp/bindings/strong_binding.h"
 #include "services/tracing/tracing.mojom.h"
 
 namespace mojo {
 
 class TraceControllerImpl : public tracing::TraceController {
  public:
   explicit TraceControllerImpl(
       InterfaceRequest<tracing::TraceController> request);
+  ~TraceControllerImpl() override;
 
-  ~TraceControllerImpl() override;
+  // Set to true if base::trace_event::TraceLog is enabled externally to this
+  // class. If this is set to true this class will save the collector but not
+  // enable tracing when it receives a StartTracing message from the tracing
+  // service.
+  void set_tracing_already_started(bool tracing_already_started) {
+    tracing_already_started_ = tracing_already_started;
+  }
 
  private:
   // tracing::TraceController implementation:
   void StartTracing(const String& categories,
                     tracing::TraceDataCollectorPtr collector) override;
   void StopTracing() override;
 
   void SendChunk(const scoped_refptr<base::RefCountedString>& events_str,
                  bool has_more_events);
 
+  bool tracing_already_started_;
   tracing::TraceDataCollectorPtr collector_;
   StrongBinding<tracing::TraceController> binding_;
 
   DISALLOW_COPY_AND_ASSIGN(TraceControllerImpl);
 };
 
 }  // namespace mojo
 
 #endif  // MOJO_COMMON_TRACING_CONTROLLER_IMPL_H_