Replace hide-scrollbars flag with a web setting.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1033 matching lines @@
     }
 
     return hasAnyScrollbar && ((shouldUseCustom != hasCustom) || (shouldUseCustom && didCustomScrollbarOwnerChanged));
 }
 
 void PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar)
 {
     if (FreezeScrollbarsScope::scrollbarsAreFrozen())
         return;
 
+    if (box().frame()->settings() && box().frame()->settings()->hideScrollbars())

[dcheng, 2016/09/19 10:04:47]

Is this null check of settings() required? We do call this during document
shutdown, but at that point, I think the Settings object should still be
non-null.

[skobes, 2016/09/19 18:37:22]

The question is whether Frame::m_host can ever be null at this point.  I am not
quite sure, but we do these checks in a lot of places and Frame::settings() has
the comment "can be null".

[skobes, 2016/09/19 19:31:20]

Ok, let's change the null checks to DCHECKs.

[Eric Seckler, 2016/09/22 12:41:56]

Done. I also changed this in FrameView, assuming it's the same reasoning there.
Does that make sense?

+        hasScrollbar = false;
+
     if (hasScrollbar == hasHorizontalScrollbar())
         return;
 
     setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
 
     m_scrollbarManager.setHasHorizontalScrollbar(hasScrollbar);
 
     updateScrollOrigin();
 
     // Destroying or creating one bar can cause our scrollbar corner to come and go. We need to update the opposite scrollbar's style.
     if (hasHorizontalScrollbar())
         horizontalScrollbar()->styleChanged();
     if (hasVerticalScrollbar())
         verticalScrollbar()->styleChanged();
 
     setScrollCornerNeedsPaintInvalidation();
 
     // Force an update since we know the scrollbars have changed things.
     if (box().document().hasAnnotatedRegions())
         box().document().setAnnotatedRegionsDirty(true);
 }
 
 void PaintLayerScrollableArea::setHasVerticalScrollbar(bool hasScrollbar)
 {
     if (FreezeScrollbarsScope::scrollbarsAreFrozen())
         return;
 
+    if (box().frame()->settings() && box().frame()->settings()->hideScrollbars())
+        hasScrollbar = false;
+
     if (hasScrollbar == hasVerticalScrollbar())
         return;
 
     setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
 
     m_scrollbarManager.setHasVerticalScrollbar(hasScrollbar);
 
     updateScrollOrigin();
 
     // Destroying or creating one bar can cause our scrollbar corner to come and go. We need to update the opposite scrollbar's style.
@@ skipping 677 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink