[MERGE #4680 @sethbrenith] Fix the build

Merge pull request #4680 from sethbrenith:user/sethb/fix-build

Fix a Field-casting issue that showed up in 1.9 and is blocking Ubuntu builds.

lib/Runtime/Debug/TTSnapObjects.cpp

@@ -2163,7 +2163,7 @@ namespace TTD
 
             Js::CallInfo callInfo(static_cast<Js::CallFlags>(generatorInfo->arguments_callInfo_flags), generatorInfo->arguments_callInfo_count, false /*unusedBool*/);
 
-            Js::Arguments arguments(callInfo, (Js::Var*)argVals);
+            Js::Arguments arguments(callInfo, unsafe_write_barrier_cast<Js::Var*>(argVals));
 
             // TODO: BUGBUG - figure out how to determine what the prototype was.  Just use GetNull() for now
             Js::RecyclableObject* prototype = ctx->GetLibrary()->GetNull();