Fix unused parameter warning (#275)

diff --git a/src/dawn/engine_dawn.cc b/src/dawn/engine_dawn.cc
index ee994f7..7bc5acd 100644
--- a/src/dawn/engine_dawn.cc
+++ b/src/dawn/engine_dawn.cc
@@ -456,8 +456,7 @@
   return {};
 }
 
-Result EngineDawn::GetFrameBuffer(std::vector<Value>* values) {
-  assert(values);
+Result EngineDawn::GetFrameBuffer(std::vector<Value>*) {
   return Result("Dawn::GetFrameBuffer not implemented");
 }