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

content/browser/devtools/protocol/tracing_handler.cc

Index: content/browser/devtools/protocol/tracing_handler.cc
diff --git a/content/browser/devtools/protocol/tracing_handler.cc b/content/browser/devtools/protocol/tracing_handler.cc
index d85695731cd3410809e28c56421b338f7b71dd0d..7eb837b388a3d46997123b48b6d403fbdcb1b886 100644
--- a/content/browser/devtools/protocol/tracing_handler.cc
+++ b/content/browser/devtools/protocol/tracing_handler.cc
@@ -8,6 +8,7 @@
 
 #include "base/bind.h"
 #include "base/format_macros.h"
+#include "base/json/json_writer.h"
 #include "base/memory/ref_counted_memory.h"
 #include "base/strings/string_split.h"
 #include "base/strings/stringprintf.h"
@@ -130,23 +131,41 @@ void TracingHandler::OnTraceToStreamComplete(const std::string& stream_handle) {
       TracingCompleteParams::Create()->set_stream(stream_handle));
 }
 
-Response TracingHandler::Start(DevToolsCommandId command_id,
-                               const std::string* categories,
-                               const std::string* options,
-                               const double* buffer_usage_reporting_interval,
-                               const std::string* transfer_mode) {
+Response TracingHandler::Start(
+    DevToolsCommandId command_id,
+    const std::string* categories,
+    const std::string* options,
+    const double* buffer_usage_reporting_interval,
+    const std::string* transfer_mode,
+    const scoped_ptr<base::DictionaryValue>& config) {
   if (IsTracing())
     return Response::InternalError("Tracing is already started");
 
+  if (config && (categories || options)) {
+    return Response::InternalError(
+        "Both trace config and categories/options are specified. Only one way "
+        "should be used; and using trace config is preferred.");
+  }
+
   did_initiate_recording_ = true;
   return_as_stream_ =
       transfer_mode && *transfer_mode == start::kTransferModeReturnAsStream;
-  base::trace_event::TraceConfig trace_config(
-      categories ? *categories : std::string(),
-      options ? *options : std::string());
   if (buffer_usage_reporting_interval)
     SetupTimer(*buffer_usage_reporting_interval);
 
+  base::trace_event::TraceConfig trace_config;
+  if (config) {
+    scoped_ptr<base::DictionaryValue> tracing_style_dict =
+        base::trace_event::TraceConfig::DevToolsToTracingStyle(config);
+    std::string trace_config_str;
+    base::JSONWriter::Write(*tracing_style_dict, &trace_config_str);

[caseq, 2016/03/05 02:25:24]

Why do we have to do an extra roundtrip of writing/parsing JSON? Perhaps, have
TraceConfig's constructor accept base::DictionaryValue?

[Primiano Tucci (use gerrit), 2016/03/06 01:24:37]

I think a good layering would be:
 -  move your dictionary conversion logic (that right now you have in base)
here.
 - I think you want a  TraceConfigFromDevtoolsConfig(scoped_ptr<DictValue>,
TraceConfig* out_trace_config) method in the anonymous namespace here.

Having to deserialize -> reserialize -> reparse JSON feels a bit... not ideal

+    trace_config = base::trace_event::TraceConfig(trace_config_str);
+  } else if (categories || options) {
+    trace_config = base::trace_event::TraceConfig(
+        categories ? *categories : std::string(),
+        options ? *options : std::string());
+  }
+
   // If inspected target is a render process Tracing.start will be handled by
   // tracing agent in the renderer.
   TracingController::GetInstance()->StartTracing(