[wasm] Improve naming consistency for WASM instances.

src/messages.h

Index: src/messages.h
diff --git a/src/messages.h b/src/messages.h
index a1994f6b59adb185eee89fa361f69622f1910ed4..b3e55cac226086e68d6ca1f3f6fe96913bee0884 100644
--- a/src/messages.h
+++ b/src/messages.h
@@ -126,7 +126,7 @@ class WasmStackFrame : public StackFrameBase {
  public:
   virtual ~WasmStackFrame() {}
 
-  Handle<Object> GetReceiver() const override { return wasm_obj_; }
+  Handle<Object> GetReceiver() const override { return wasm_instance_; }
   Handle<Object> GetFunction() const override;
 
   Handle<Object> GetFileName() override { return Null(); }
@@ -153,7 +153,7 @@ class WasmStackFrame : public StackFrameBase {
 
   Isolate* isolate_;
 
-  Handle<Object> wasm_obj_;
+  Handle<Object> wasm_instance_;
   uint32_t wasm_func_index_;
   Handle<AbstractCode> code_;
   int offset_;