Fix performance regression and bug with hide_scrollbars setting.

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

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 3433 matching lines @@
   return maximumPosition.expandedTo(minimumScrollPosition());
 }
 
 void FrameView::addChild(Widget* child) {
   ASSERT(child != this && !child->parent());
   child->setParent(this);
   m_children.add(child);
 }
 
 void FrameView::setHasHorizontalScrollbar(bool hasBar) {
-  if (m_frame->settings() && m_frame->settings()->hideScrollbars())
-    hasBar = false;
-
   if (hasBar == !!m_horizontalScrollbar)
     return;
 
   if (hasBar) {
     m_horizontalScrollbar = createScrollbar(HorizontalScrollbar);
     addChild(m_horizontalScrollbar.get());
     didAddScrollbar(*m_horizontalScrollbar, HorizontalScrollbar);
     m_horizontalScrollbar->styleChanged();
   } else {
     willRemoveScrollbar(*m_horizontalScrollbar, HorizontalScrollbar);
     // If the scrollbar has been marked as overlapping the window resizer,
     // then its removal should reduce the count.
     if (m_horizontalScrollbar->overlapsResizer())
       adjustScrollbarsAvoidingResizerCount(-1);
     removeChild(m_horizontalScrollbar.get());
     m_horizontalScrollbar->disconnectFromScrollableArea();
     m_horizontalScrollbar = nullptr;
   }
 
   setScrollCornerNeedsPaintInvalidation();
 }
 
 void FrameView::setHasVerticalScrollbar(bool hasBar) {
-  if (m_frame->settings() && m_frame->settings()->hideScrollbars())
-    hasBar = false;
-
   if (hasBar == !!m_verticalScrollbar)
     return;
 
   if (hasBar) {
     m_verticalScrollbar = createScrollbar(VerticalScrollbar);
     addChild(m_verticalScrollbar.get());
     didAddScrollbar(*m_verticalScrollbar, VerticalScrollbar);
     m_verticalScrollbar->styleChanged();
   } else {
     willRemoveScrollbar(*m_verticalScrollbar, VerticalScrollbar);
@@ skipping 191 matching lines @@
   return (m_horizontalScrollbar &&
           m_horizontalScrollbar->isOverlayScrollbar()) ||
          (m_verticalScrollbar && m_verticalScrollbar->isOverlayScrollbar());
 }
 
 void FrameView::computeScrollbarExistence(
     bool& newHasHorizontalScrollbar,
     bool& newHasVerticalScrollbar,
     const IntSize& docSize,
     ComputeScrollbarExistenceOption option) const {
+  if (m_frame->settings() && m_frame->settings()->hideScrollbars()) {
+    newHasHorizontalScrollbar = false;
+    newHasVerticalScrollbar = false;
+    return;
+  }
+
   bool hasHorizontalScrollbar = m_horizontalScrollbar;
   bool hasVerticalScrollbar = m_verticalScrollbar;
 
   newHasHorizontalScrollbar = hasHorizontalScrollbar;
   newHasVerticalScrollbar = hasVerticalScrollbar;
 
   if (RuntimeEnabledFeatures::rootLayerScrollingEnabled())
     return;
 
   ScrollbarMode hScroll = m_horizontalScrollbarMode;
@@ skipping 866 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const {
   if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
     return false;
   return m_subtreeThrottled ||
          (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 }  // namespace blink