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

base/trace_event/trace_config.h

Index: base/trace_event/trace_config.h
diff --git a/base/trace_event/trace_config.h b/base/trace_event/trace_config.h
index 81b7d515133dd0c3c71f461316ced768e6fd9618..8ea249502d641919adbda0fb620f58814fffa803 100644
--- a/base/trace_event/trace_config.h
+++ b/base/trace_event/trace_config.h
@@ -130,6 +130,9 @@ class BASE_EXPORT TraceConfig {
   // disabled-by-default-memory-infra category is enabled.
   explicit TraceConfig(const std::string& config_string);
 
+  // Create TraceConfig object from the trace config dictionary.

[Primiano Tucci (use gerrit), 2016/03/14 16:53:02]

I would probably reword this comment as:
// Functionally identical to the above, but takes a parsed dictionary as input
instead of its JSON serialization.

[Zhen Wang, 2016/03/14 21:41:45]

Done.

+  explicit TraceConfig(const base::DictionaryValue& config);

[Primiano Tucci (use gerrit), 2016/03/14 16:53:02]

nit: no need of base::, you are already in namespace base :)

[Primiano Tucci (use gerrit), 2016/03/14 16:53:02]

Since you introduced this, I think you can make a 2 line fix to
components/tracing/trace_config_file.cc.
That one is creating a DictionaryValue and serializing to JSON. THe latter does
not need to happen anymore thanks to your CL

[Primiano Tucci (use gerrit), 2016/03/14 16:53:02]

Can we haz a unittest to cover this casE?
I think you want to refactor some of the existing ones, so you can call them
either from the string or dictionary.

[Zhen Wang, 2016/03/14 21:41:45]

Updated trace_config_file.cc and trace_config_unittest.cc

+
   TraceConfig(const TraceConfig& tc);
 
   ~TraceConfig();
@@ -190,7 +193,10 @@ class BASE_EXPORT TraceConfig {
   // in the suffix 'Debug' or 'Test'.
   void InitializeDefault();
 
-  // Initialize from the config string
+  // Initialize from the config dictionary.

[petrcermak, 2016/03/14 18:05:09]

supernit: s/the/a/

[Zhen Wang, 2016/03/14 21:41:45]

Done.

+  void InitializeFromConfigDict(const base::DictionaryValue& dict);

[Primiano Tucci (use gerrit), 2016/03/14 16:53:02]

nit, ditto: no need of base::

[Zhen Wang, 2016/03/14 21:41:45]

Done.

+
+  // Initialize from the config string.

[petrcermak, 2016/03/14 18:05:09]

ditto (s/the/a/)

[Zhen Wang, 2016/03/14 21:41:45]

Done.

   void InitializeFromConfigString(const std::string& config_string);
 
   // Initialize from category filter and trace options strings