Moving prototype from Google to GitHub

I'd started some prototyping work on this change before the move to
GitHub; this change restores things.
diff --git a/src/core/surface/byte_buffer.c b/src/core/surface/byte_buffer.c
index 27a6c6e..4087622 100644
--- a/src/core/surface/byte_buffer.c
+++ b/src/core/surface/byte_buffer.c
@@ -49,6 +49,16 @@
   return bb;
 }
 
+grpc_byte_buffer *grpc_byte_buffer_copy(grpc_byte_buffer *bb) {
+  switch (bb->type) {
+    case GRPC_BB_SLICE_BUFFER:
+      return grpc_byte_buffer_create(bb->data.slice_buffer.slices, bb->data.slice_buffer.count);
+  }
+  gpr_log(GPR_INFO, "should never get here");
+  abort();
+  return NULL;
+}
+
 void grpc_byte_buffer_destroy(grpc_byte_buffer *bb) {
   switch (bb->type) {
     case GRPC_BB_SLICE_BUFFER: