Revert of Remove FrameView::Root (patchset #2 id:20001 of...

Revert of Remove FrameView::Root (patchset #2 id:20001 of https://codereview.chromium.org/2849383002/ )

Reason for revert:
This broke compilation (details later).

Today's sheriff

Original issue's description:
> Remove FrameView::Root
>
> BUG=637460
>
> Review-Url: https://codereview.chromium.org/2849383002
> Cr-Commit-Position: refs/heads/master@{#468588}
> Committed: https://chromium.googlesource.com/chromium/src/+/561e38ff8dc24f23f0293c54777bbc2eda99a2b5

TBR=haraken@chromium.org,dcheng@chromium.org,joelhockey@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=637460

Review-Url: https://codereview.chromium.org/2857523002
Cr-Commit-Position: refs/heads/master@{#468589}

third_party/WebKit/Source/core/frame/FrameView.cpp

@@ -3848,6 +3848,13 @@ void FrameView::RemoveAnimatingScrollableArea(ScrollableArea* scrollable_area) {
   animating_scrollable_areas_->erase(scrollable_area);
 }
 
+FrameView* FrameView::Root() const {
+  const FrameView* top = this;
+  while (top->Parent())
+    top = ToFrameView(top->Parent());
+  return const_cast<FrameView*>(top);
+}
+
 void FrameView::SetParent(FrameViewBase* parent_frame_view_base) {
   FrameView* parent = ToFrameView(parent_frame_view_base);
   if (parent == parent_)

third_party/WebKit/Source/core/frame/FrameView.h

@@ -498,6 +498,7 @@ class CORE_EXPORT FrameView final
   void SetSelfVisible(bool v) { self_visible_ = v; }
   void SetParent(FrameViewBase*) override;
   FrameViewBase* Parent() const override { return parent_; }
+  FrameView* Root() const;
   void RemoveChild(FrameViewBase*);
   void AddChild(FrameViewBase*);
   const ChildrenSet* Children() const { return &children_; }

third_party/WebKit/Source/web/WebInputEventConversion.cpp

@@ -51,7 +51,7 @@ namespace {
 float FrameScale(const FrameView* frame_view) {
   float scale = 1;
   if (frame_view) {
-    FrameView* root_view = frame_view->GetFrame().LocalFrameRoot()->View();
+    FrameView* root_view = frame_view->Root();
     if (root_view)
       scale = root_view->InputEventsScaleFactor();
   }
@@ -64,7 +64,7 @@ FloatPoint FrameTranslation(const FrameView* frame_view) {
   IntPoint visual_viewport;
   FloatSize overscroll_offset;
   if (frame_view) {
-    FrameView* root_view = frame_view->GetFrame().LocalFrameRoot()->View();
+    FrameView* root_view = frame_view->Root();
     if (root_view) {
       scale = root_view->InputEventsScaleFactor();
       offset = FloatSize(root_view->InputEventsOffsetForEmulation());