[wasm] Differentiate unnamed and empty names

src/frames.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_FRAMES_H_
 #define V8_FRAMES_H_
 
 #include "src/allocation.h"
 #include "src/handles.h"
 #include "src/safepoint-table.h"
@@ skipping 953 matching lines @@
   // Printing support.
   void Print(StringStream* accumulator, PrintMode mode,
              int index) const override;
 
   // Determine the code for the frame.
   Code* unchecked_code() const override;
 
   Object* wasm_obj();
   uint32_t function_index();
 
-  Object* function_name();
+  // The function name, or nullptr if the function is unnamed, the name is not a
+  // valid UTF-8 string, or no wasm_obj is available (in testing).
+  String* function_name();
 
   static WasmFrame* cast(StackFrame* frame) {
     DCHECK(frame->is_wasm());
     return static_cast<WasmFrame*>(frame);
   }
 
  protected:
   inline explicit WasmFrame(StackFrameIteratorBase* iterator);
 
   Address GetCallerStackPointer() const override;
@@ skipping 235 matching lines @@
 
 
 // Reads all frames on the current stack and copies them into the current
 // zone memory.
 Vector<StackFrame*> CreateStackMap(Isolate* isolate, Zone* zone);
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_FRAMES_H_