[WIP] A sampler thread in d8 for consuming the new API.

src/log-utils.cc

Index: src/log-utils.cc
diff --git a/src/log-utils.cc b/src/log-utils.cc
index 8f012252fce9d9252eefe64c677da132de71cf9a..d9b6e53ce1c009747df3b463a9c01f541139288f 100644
--- a/src/log-utils.cc
+++ b/src/log-utils.cc
@@ -217,6 +217,23 @@ void Log::MessageBuilder::AppendStringPart(const char* str, int len) {
 }
 
 
+void Log::MessageBuilder::StartSubstring() {
+  substr_start_pos_ = pos_;
+}
+
+
+void Log::MessageBuilder::EndSubstring() {
+  substr_end_pos_ = pos_;
+}
+
+
+void Log::MessageBuilder::GetSubstring(std::string* str) {
+  ASSERT_EQ(0, str->size());
+  str->append(log_->message_buffer_ + substr_start_pos_,
+              substr_end_pos_ - substr_start_pos_);
+}
+
+
 void Log::MessageBuilder::WriteToLogFile() {
   ASSERT(pos_ <= Log::kMessageBufferSize);
   const int written = log_->WriteToFile(log_->message_buffer_, pos_);