Reland - 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..75e001b275974ef0cfae4230485d029cd68a82f2 100644
--- a/base/trace_event/trace_config.h
+++ b/base/trace_event/trace_config.h
@@ -130,6 +130,10 @@ class BASE_EXPORT TraceConfig {
   // disabled-by-default-memory-infra category is enabled.
   explicit TraceConfig(const std::string& config_string);
 
+  // Functionally identical to the above, but takes a parsed dictionary as input
+  // instead of its JSON serialization.
+  explicit TraceConfig(const DictionaryValue& config);
+
   TraceConfig(const TraceConfig& tc);
 
   ~TraceConfig();
@@ -190,7 +194,10 @@ class BASE_EXPORT TraceConfig {
   // in the suffix 'Debug' or 'Test'.
   void InitializeDefault();
 
-  // Initialize from the config string
+  // Initialize from a config dictionary.
+  void InitializeFromConfigDict(const DictionaryValue& dict);
+
+  // Initialize from a config string.
   void InitializeFromConfigString(const std::string& config_string);
 
   // Initialize from category filter and trace options strings