message: Add an additional StartTracing overload that allows implementations of
  Tracing::Engine to avoid unnecessary string copies.
diff: @@ -166,10 +166,14 @@ class Tracing::Engine {
  // May return nullptr if annotations are not supported.
  virtual Annotation* PushAnnotation(StringPiece name) = 0;
  - // Start tracing under the specified label. Caller should delete the
  - // result to stop tracing.
  + // Start tracing under the specified label. Caller should delete the result
  + // to stop tracing.
  // May return nullptr if tracing is not supported.
  virtual Tracer* StartTracing(StringPiece label) = 0;
  + // Same as above, but implementations can avoid copying the string.
  + virtual Tracer* StartTracing(string&& label) {
  + return StartTracing(StringPiece(label));
  + }
  };
  // This class permits a user to apply annotation on kernels and memcpys
  