[tracing] Make TracedValue accept const char* strings

This should speed up trace generation for cpu profile as it eliminates
creation of temporary std::string objects.

Review-Url: https://codereview.chromium.org/2555043003
Cr-Commit-Position: refs/heads/master@{#41736}

src/tracing/traced-value.cc

@@ -23,40 +23,36 @@ const bool kStackTypeArray = true;
 #define DEBUG_POP_CONTAINER() ((void)0)
 #endif
 
-std::string EscapeString(const std::string& value) {
-  std::string result;
-  result.reserve(value.length() + 2);
-  result += '"';
-  size_t length = value.length();
+void EscapeAndAppendString(const char* value, std::string* result) {
+  *result += '"';
   char number_buffer[10];
-  for (size_t src = 0; src < length; ++src) {
-    char c = value[src];
+  while (*value) {
+    char c = *value++;
     switch (c) {
       case '\t':
-        result += "\\t";
+        *result += "\\t";
         break;
       case '\n':
-        result += "\\n";
+        *result += "\\n";
         break;
       case '\"':
-        result += "\\\"";
+        *result += "\\\"";
         break;
       case '\\':
-        result += "\\\\";
+        *result += "\\\\";
         break;
       default:
         if (c < '\040') {
           base::OS::SNPrintF(
               number_buffer, arraysize(number_buffer), "\\u%04X",
               static_cast<unsigned>(static_cast<unsigned char>(c)));
-          result += number_buffer;
+          *result += number_buffer;
         } else {
-          result += c;
+          *result += c;
         }
     }
   }
-  result += '"';
-  return result;
+  *result += '"';
 }
 
 }  // namespace
@@ -93,10 +89,10 @@ void TracedValue::SetBoolean(const char* name, bool value) {
   data_ += value ? "true" : "false";
 }
 
-void TracedValue::SetString(const char* name, const std::string& value) {
+void TracedValue::SetString(const char* name, const char* value) {
   DCHECK_CURRENT_CONTAINER_IS(kStackTypeDict);
   WriteName(name);
-  data_ += EscapeString(value);
+  EscapeAndAppendString(value, &data_);
 }
 
 void TracedValue::BeginDictionary(const char* name) {
@@ -133,10 +129,10 @@ void TracedValue::AppendBoolean(bool value) {
   data_ += value ? "true" : "false";
 }
 
-void TracedValue::AppendString(const std::string& value) {
+void TracedValue::AppendString(const char* value) {
   DCHECK_CURRENT_CONTAINER_IS(kStackTypeArray);
   WriteComma();
-  data_ += EscapeString(value);
+  EscapeAndAppendString(value, &data_);
 }
 
 void TracedValue::BeginDictionary() {

src/tracing/traced-value.h

@@ -29,14 +29,18 @@ class TracedValue : public ConvertableToTraceFormat {
   void SetInteger(const char* name, int value);
   void SetDouble(const char* name, double value);
   void SetBoolean(const char* name, bool value);
-  void SetString(const char* name, const std::string& value);
+  void SetString(const char* name, const char* value);
+  void SetString(const char* name, const std::string& value) {
+    SetString(name, value.c_str());
+  }
   void BeginDictionary(const char* name);
   void BeginArray(const char* name);
 
   void AppendInteger(int);
   void AppendDouble(double);
   void AppendBoolean(bool);
-  void AppendString(const std::string&);
+  void AppendString(const char*);
+  void AppendString(const std::string& value) { AppendString(value.c_str()); }
   void BeginArray();
   void BeginDictionary();