[Tracing] Remove unnecessary memory allocation in runtime call stats.

src/counters.cc

Index: src/counters.cc
diff --git a/src/counters.cc b/src/counters.cc
index 84568ac351fc696db2638d106f9d3b3fa02c9c41..c4e86460aa1140054a09f1f0abc0a1350e4054da 100644
--- a/src/counters.cc
+++ b/src/counters.cc
@@ -364,7 +364,7 @@ void RuntimeCallStats::Reset() {
   in_use_ = true;
 }
 
-const char* RuntimeCallStats::Dump() {
+std::string RuntimeCallStats::Dump() {
   buffer_.str(std::string());
   buffer_.clear();
   buffer_ << "{";
@@ -393,15 +393,8 @@ const char* RuntimeCallStats::Dump() {
   FOR_EACH_HANDLER_COUNTER(DUMP_COUNTER)
 #undef DUMP_COUNTER
   buffer_ << "\"END\":[]}";
-  const std::string& buffer_str = buffer_.str();
-  size_t length = buffer_str.size();
-  if (length > len_) {
-    buffer_c_str_.reset(new char[length + 1]);
-    len_ = length;
-  }
-  strncpy(buffer_c_str_.get(), buffer_str.c_str(), length + 1);
   in_use_ = false;
-  return buffer_c_str_.get();
+  return buffer_.str();
 }
 
 }  // namespace internal