[WebPayments] Factor out sheet-specific logic in Controllers.

chrome/browser/ui/views/payments/view_stack.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/views/payments/view_stack.h"
 
 #include "ui/views/layout/fill_layout.h"
 
 ViewStack::ViewStack()
     : slide_in_animator_(base::MakeUnique<views::BoundsAnimator>(this)),
       slide_out_animator_(base::MakeUnique<views::BoundsAnimator>(this)) {
   SetLayoutManager(new views::FillLayout());
 
   slide_out_animator_->AddObserver(this);
+  // Paint to a layer and Mask to Bounds, otherwise descendant views that paint
+  // to a layer themselves will still paint while they're being animated out and
+  // are out of bounds of their parent.
+  SetPaintToLayer(true);
+  layer()->SetMasksToBounds(true);
 }
 
 ViewStack::~ViewStack() {}
 
 void ViewStack::Push(std::unique_ptr<views::View> view, bool animate) {
   gfx::Rect destination = bounds();
   destination.set_origin(gfx::Point(0, 0));
+
   if (animate) {
     // First add the new view out of bounds since it'll slide in from right to
     // left.
     view->SetBounds(width(), 0, width(), height());
     view->Layout();
 
     AddChildView(view.get());
 
     // Animate the new view to be right on top of this one.
     slide_in_animator_->AnimateViewTo(view.get(), destination);
@@ skipping 51 matching lines @@
 }
 
 void ViewStack::OnBoundsAnimatorDone(views::BoundsAnimator* animator) {
   // This should only be called from slide_out_animator_ when the views going
   // out are done animating.
   DCHECK_EQ(animator, slide_out_animator_.get());
 
   stack_.pop_back();
   DCHECK(!stack_.empty()) << "State stack should never be empty";
 }