[classes] Set proper representation during fast class boilerplate instantiation.

Bug: chromium:791368
Change-Id: I86d9df38698d9c8b6109d0a11579fa28810ba1dc
Reviewed-on: https://chromium-review.googlesource.com/833908Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50206}

src/runtime/runtime-classes.cc

@@ -310,6 +310,9 @@ bool AddDescriptorsByTemplate(
           value = GetMethodWithSharedNameAndSetHomeObject(isolate, args, value,
                                                           *receiver);
         }
+        details =
+            details.CopyWithRepresentation(value->OptimalRepresentation());
+
       } else {
         DCHECK_EQ(kAccessor, details.kind());
         if (value->IsAccessorPair()) {
@@ -330,6 +333,7 @@ bool AddDescriptorsByTemplate(
       DCHECK_EQ(kField, details.location());
       DCHECK(!details.representation().IsDouble());
     }
+    DCHECK(value->FitsRepresentation(details.representation()));
     descriptors->Set(i, name, value, details);
   }