Tidy PDF_ENABLE_XFA #ifdefs in fpdfsdk.

-- label matching #endifs
-- prefer #ifdef over #ifndef
-- consolidate some blocks.

R=ochang@chromium.org

Review URL: https://codereview.chromium.org/1484843002 .
diff --git a/fpdfsdk/src/javascript/JS_Runtime.h b/fpdfsdk/src/javascript/JS_Runtime.h
index 6de2b70..decc553 100644
--- a/fpdfsdk/src/javascript/JS_Runtime.h
+++ b/fpdfsdk/src/javascript/JS_Runtime.h
@@ -59,13 +59,12 @@
   v8::Local<v8::Context> NewJSContext();
 
 #ifdef PDF_ENABLE_XFA
-  // IJS_Runtime:
   FX_BOOL GetHValueByName(const CFX_ByteStringC& utf8Name,
                           FXJSE_HVALUE hValue) override;
   FX_BOOL SetHValueByName(const CFX_ByteStringC& utf8Name,
                           FXJSE_HVALUE hValue) override;
+#endif  // PDF_ENABLE_XFA
 
-#endif
   void AddObserver(Observer* observer);
   void RemoveObserver(Observer* observer);