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

Previously we didn't implement TRACE_STR_COPY when we write trace events to
file, which causes us to allocate a growing independent memory chunk for dumped
runtime call stats table. Since we now have a fully functional TRACE_STR_COPY,
this memory allocation can be avoided, this patch removes it.

BUG=v8:5089

Review-Url: https://codereview.chromium.org/2342643004
Cr-Commit-Position: refs/heads/master@{#39462}

src/counters.cc

@@ -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().c_str();
 }
 
 }  // namespace internal

src/counters.h

@@ -803,8 +803,6 @@ class RuntimeCallStats {
 
  private:
   std::stringstream buffer_;
-  std::unique_ptr<char[]> buffer_c_str_;
-  size_t len_ = 0;
   // Counter to track recursive time events.
   RuntimeCallTimer* current_timer_ = NULL;
   // Used to track nested tracing scopes.