Replace Node& cast with Member<T>::Get()

This came from https://crrev.com/c/559933. The cast was added to satisfy
a Windows compile error as it couldn't resolve the arguments to
operator!=.

A less ugly fix is to just get the raw pointer out of Member.

Bug: 
Change-Id: I14ed38a936aaf0457c2001a5e2c75b1a5ed91ac5
Reviewed-on: https://chromium-review.googlesource.com/567171Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486211}

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp

@@ -223,11 +223,11 @@ bool RootScrollerController::ScrollsViewport(const Element& element) const {
 }
 
 void RootScrollerController::ElementRemoved(const Element& element) {
-  if ((Node&)element != effective_root_scroller_)
+  if (element != effective_root_scroller_.Get())
     return;
 
   RecomputeEffectiveRootScroller();
-  DCHECK((Node&)element != effective_root_scroller_);
+  DCHECK(element != effective_root_scroller_.Get());
 }
 
 }  // namespace blink