Adding a ToPrettyString method to ConvertableToTracedValue.

base/debug/trace_event_argument.cc

Index: base/debug/trace_event_argument.cc
diff --git a/base/debug/trace_event_argument.cc b/base/debug/trace_event_argument.cc
index 90e924f119e84352f79945360407219518e3f470..a246b6b784cb946a6c2e7aa330ee4c74b33cd104 100644
--- a/base/debug/trace_event_argument.cc
+++ b/base/debug/trace_event_argument.cc
@@ -113,5 +113,15 @@ void TracedValue::AppendAsTraceFormat(std::string* out) const {
   DCHECK_EQ(1u, stack_.size()) << tmp;
 }
 
+std::string TracedValue::ToPrettyString() const {
+  std::string tmp;
+  JSONWriter::WriteWithOptions(
+      stack_.front(),
+      JSONWriter::OPTIONS_PRETTY_PRINT | JSONWriter::OPTIONS_OMIT_BINARY_VALUES,
+      &tmp);
+  DCHECK_EQ(1u, stack_.size()) << tmp;
+  return tmp;
+}
+
 }  // namespace debug
 }  // namespace base