Update DevTools Tracing.Start to accept trace config as a parameter

components/tracing/trace_config_file.cc

Index: components/tracing/trace_config_file.cc
diff --git a/components/tracing/trace_config_file.cc b/components/tracing/trace_config_file.cc
index 670bcc9c2064d52257d973c4d7a1a9878812d85b..e0d0883ebadcfa1c221c101467a0262a3e03747c 100644
--- a/components/tracing/trace_config_file.cc
+++ b/components/tracing/trace_config_file.cc
@@ -11,7 +11,6 @@
 #include "base/command_line.h"
 #include "base/files/file_util.h"
 #include "base/json/json_reader.h"
-#include "base/json/json_writer.h"
 #include "base/logging.h"
 #include "base/memory/singleton.h"
 #include "base/values.h"
@@ -108,9 +107,8 @@ bool TraceConfigFile::ParseTraceConfigFileContent(const std::string& content) {
   if (!dict->GetDictionary(kTraceConfigParam, &trace_config_dict))
     return false;
 
-  std::string trace_config_str;
-  base::JSONWriter::Write(*trace_config_dict, &trace_config_str);
-  trace_config_ = base::trace_event::TraceConfig(trace_config_str);
+  DCHECK(trace_config_dict);

[Primiano Tucci (use gerrit), 2016/03/15 12:22:41]

If you dereference a ptr there is no point adding a DCHECK, your code will crash
on the next line anyways, DCHECK does not add anything useful.
It's different if you were storing a pointer and reusing it later.

[Zhen Wang, 2016/03/15 16:53:55]

I see. removed. 

+  trace_config_ = base::trace_event::TraceConfig(*trace_config_dict);
 
   if (!dict->GetInteger(kStartupDurationParam, &startup_duration_))
       startup_duration_ = 0;