[cleanup] Make SFI::instance_class_name a String

Make the accessors of SharedFunctionInfo instance_class_name take and
return String rather than Object, since it's always a String anyway.

Change-Id: Ic5dacccf3835550e3533356fe7ded37ea107d720
Reviewed-on: https://chromium-review.googlesource.com/774882Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49421}

src/objects.cc

@@ -1337,7 +1337,9 @@ Handle<SharedFunctionInfo> FunctionTemplateInfo::GetOrCreateSharedFunctionInfo(
   }
 
   result->set_length(info->length());
-  if (class_name->IsString()) result->set_instance_class_name(*class_name);
+  if (class_name->IsString()) {
+    result->set_instance_class_name(String::cast(*class_name));
+  }
   result->set_api_func_data(*info);
   result->DontAdaptArguments();
   DCHECK(result->IsApiFunction());

src/objects/shared-function-info-inl.h

@@ -27,7 +27,7 @@ ACCESSORS(SharedFunctionInfo, raw_name, Object, kNameOffset)
 ACCESSORS(SharedFunctionInfo, construct_stub, Code, kConstructStubOffset)
 ACCESSORS(SharedFunctionInfo, feedback_metadata, FeedbackMetadata,
           kFeedbackMetadataOffset)
-ACCESSORS(SharedFunctionInfo, instance_class_name, Object,
+ACCESSORS(SharedFunctionInfo, instance_class_name, String,
           kInstanceClassNameOffset)
 ACCESSORS(SharedFunctionInfo, function_data, Object, kFunctionDataOffset)
 ACCESSORS(SharedFunctionInfo, script, Object, kScriptOffset)

src/objects/shared-function-info.h

@@ -137,7 +137,7 @@ class SharedFunctionInfo : public HeapObject {
 #endif
 
   // [instance class name]: class name for instances.
-  DECL_ACCESSORS(instance_class_name, Object)
+  DECL_ACCESSORS(instance_class_name, String)
 
   // [function data]: This field holds some additional data for function.
   // Currently it has one of: