Add builders for tracing event's structural arguments

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 7b191b8a403f2bbf7baf25bde779ba691478db26..146ca8f2aa55b545749e4d6cc520c8bc30431ccd 100644
--- a/base/debug/trace_event_impl.h
+++ b/base/debug/trace_event_impl.h
@@ -64,7 +64,8 @@ namespace debug {
 
 // For any argument of type TRACE_VALUE_TYPE_CONVERTABLE the provided
 // class must implement this interface.
-class ConvertableToTraceFormat : public RefCounted<ConvertableToTraceFormat> {
+class BASE_EXPORT ConvertableToTraceFormat
+    : public RefCounted<ConvertableToTraceFormat> {
  public:
   // Append the class info to the provided |out| string. The appended
   // data must be a valid JSON object. Strings must be properly quoted, and
@@ -72,6 +73,12 @@ class ConvertableToTraceFormat : public RefCounted<ConvertableToTraceFormat> {
   // appended.
   virtual void AppendAsTraceFormat(std::string* out) const = 0;
 
+  std::string ToString() const {
+    std::string result;
+    AppendAsTraceFormat(&result);
+    return result;
+  }
+
  protected:
   virtual ~ConvertableToTraceFormat() {}