CodeGen: Move FillMembersInternal to protected

It is called from derived classes' FillMembersInternal.

Bug: 839389
Change-Id: Icc214c548d8f02da13a594369c015043d4118593
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026630Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736234}

third_party/blink/renderer/bindings/scripts/bind_gen/dictionary.py

@@ -558,6 +558,11 @@ bool FillWithMembers(
     v8::Isolate* isolate,
     v8::Local<v8::Object> creation_context,
     v8::Local<v8::Object> v8_dictionary) const override;
+
+void FillMembersInternal(
+    v8::Isolate* isolate,
+    v8::Local<v8::Object> v8_dictionary,
+    ExceptionState& exception_state);
 """))
 
     private_section = class_def.private_section
@@ -574,10 +579,6 @@ void FillMembers(
     v8::Isolate* isolate,
     v8::Local<v8::Object> v8_dictionary,
     ExceptionState& exception_state);
-void FillMembersInternal(
-    v8::Isolate* isolate,
-    v8::Local<v8::Object> v8_dictionary,
-    ExceptionState& exception_state);
 """))
 
     # C++ member variables for values