Add a ConvertableToTraceFormat type to the trace framework.

base/debug/trace_event_impl.h

Index: base/debug/trace_event_impl.h
diff --git a/base/debug/trace_event_impl.h b/base/debug/trace_event_impl.h
index 7652419e1e03c4510bd3ade5c1037b6313588734..d9f8f1ccdf04c433a5f8a973be2eb8943271fcbb 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -46,6 +46,19 @@ class WaitableEvent;
 
 namespace debug {
 
+// For any argument of type TRACE_VALUE_TYPE_CONVERTABLE the provided
+// class must implement this interface.
+class ConvertableToTraceFormat {
+ public:
+  virtual ~ConvertableToTraceFormat() {}
+
+  // Append the class info to the provided |out| string. The appended
+  // data must be a valid JSON object. Strings must be propertly quoted, and
+  // escaped. There is no processing applied to the content after it is
+  // appended.
+  virtual void AppendAsTraceFormat(std::string* out) const = 0;
+};
+
 const int kTraceMaxNumArgs = 2;
 
 // Output records are "Events" and can be obtained via the
@@ -74,7 +87,10 @@ class BASE_EXPORT TraceEvent {
              const char** arg_names,
              const unsigned char* arg_types,
              const unsigned long long* arg_values,
+             scoped_ptr<ConvertableToTraceFormat> convertable_values[],
              unsigned char flags);
+  TraceEvent(const TraceEvent& other);
+  TraceEvent& operator=(const TraceEvent& other);
   ~TraceEvent();
 
   // Serialize event data to JSON
@@ -106,6 +122,7 @@ class BASE_EXPORT TraceEvent {
   unsigned long long id_;
   TraceValue arg_values_[kTraceMaxNumArgs];
   const char* arg_names_[kTraceMaxNumArgs];
+  scoped_ptr<ConvertableToTraceFormat> convertable_values_[kTraceMaxNumArgs];
   const unsigned char* category_enabled_;
   const char* name_;
   scoped_refptr<base::RefCountedString> parameter_copy_storage_;
@@ -318,6 +335,7 @@ class BASE_EXPORT TraceLog {
                      const char** arg_names,
                      const unsigned char* arg_types,
                      const unsigned long long* arg_values,
+                     scoped_ptr<ConvertableToTraceFormat> convertable_values[],
                      unsigned char flags);
   void AddTraceEventWithThreadIdAndTimestamp(
       char phase,
@@ -330,6 +348,7 @@ class BASE_EXPORT TraceLog {
       const char** arg_names,
       const unsigned char* arg_types,
       const unsigned long long* arg_values,
+      scoped_ptr<ConvertableToTraceFormat> convertable_values[],
       unsigned char flags);
   static void AddTraceEventEtw(char phase,
                                const char* name,