Return v8::Date specialization not v8::Value where possible

Also get rid of FXJS_ValueCopy() while we're at it.

BUG=pdfium:556

Review-Url: https://codereview.chromium.org/2215093002
diff --git a/fpdfsdk/javascript/JS_Value.h b/fpdfsdk/javascript/JS_Value.h
index 5943aaf..01b4e5b 100644
--- a/fpdfsdk/javascript/JS_Value.h
+++ b/fpdfsdk/javascript/JS_Value.h
@@ -159,7 +159,7 @@
            int sec);
   virtual ~CJS_Date();
 
-  void Attach(v8::Local<v8::Value> pDate);
+  void Attach(v8::Local<v8::Date> pDate);
   bool IsValidDate() const;
 
   int GetYear() const;
@@ -186,7 +186,7 @@
   CFX_WideString ToString() const;
 
  protected:
-  v8::Local<v8::Value> m_pDate;
+  v8::Local<v8::Date> m_pDate;
   CJS_Runtime* const m_pJSRuntime;
 };