Add timeline stream control and trace retrieval to the dart embedder api.

runtime/vm/json_stream.h

Index: runtime/vm/json_stream.h
diff --git a/runtime/vm/json_stream.h b/runtime/vm/json_stream.h
index e581560d75932cbf2a0daf7f70c99570d1ab1c32..119e1291915e6af81d2f910452c6421caf003986 100644
--- a/runtime/vm/json_stream.h
+++ b/runtime/vm/json_stream.h
@@ -77,6 +77,8 @@ class JSONStream : ValueObject {
   TextBuffer* buffer() { return &buffer_; }
   const char* ToCString() { return buffer_.buf(); }
 
+  void Steal(const char** buffer, intptr_t* buffer_length);
+
   void set_reply_port(Dart_Port port);
 
   void SetParams(const char** param_keys, const char** param_values,