Hide non-composited native scrollbars.

third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 278 matching lines @@
     } else {
         thumbRect = IntRect(trackRect.x(), trackRect.y() + thumbPos, scrollbar.width(), thumbLength(scrollbar));
         beforeThumbRect = IntRect(trackRect.x(), trackRect.y(), trackRect.width(), thumbPos + thumbRect.height() / 2);
         afterThumbRect = IntRect(trackRect.x(), trackRect.y() + beforeThumbRect.height(), trackRect.width(), trackRect.maxY() - beforeThumbRect.maxY());
     }
 }
 
 ScrollbarTheme& ScrollbarTheme::theme()
 {
     if (ScrollbarTheme::mockScrollbarsEnabled()) {
+        if (RuntimeEnabledFeatures::hideScrollbarsEnabled()) {
+            DEFINE_STATIC_LOCAL(ScrollbarThemeOverlayMock, hiddenMockTheme, (ScrollbarThemeOverlayMock::HideScrollbars));
+            return hiddenMockTheme;
+        }
         if (RuntimeEnabledFeatures::overlayScrollbarsEnabled()) {
             DEFINE_STATIC_LOCAL(ScrollbarThemeOverlayMock, overlayMockTheme, ());
             return overlayMockTheme;
         }
 
         DEFINE_STATIC_LOCAL(ScrollbarThemeMock, mockTheme, ());
         return mockTheme;
     }
+    if (RuntimeEnabledFeatures::overlayScrollbarsEnabled() && RuntimeEnabledFeatures::hideScrollbarsEnabled()) {

[skobes, 2016/08/12 18:29:54]

Do we need to check overlayScrollbarsEnabled(), or is it implied by
hideScrollbarsEnabled()?

+        DEFINE_STATIC_LOCAL(ScrollbarThemeOverlay, hiddenTheme, (0, 0, ScrollbarThemeOverlay::DisallowHitTest, Color()));
+        return hiddenTheme;
+    }
     return nativeTheme();
 }
 
 void ScrollbarTheme::setMockScrollbarsEnabled(bool flag)
 {
     gMockScrollbarsEnabled = flag;
 }
 
 bool ScrollbarTheme::mockScrollbarsEnabled()
 {
@@ skipping 24 matching lines @@
         return DisplayItem::ScrollbarBackTrack;
     case ForwardTrackPart:
         return DisplayItem::ScrollbarForwardTrack;
     default:
         ASSERT_NOT_REACHED();
         return DisplayItem::ScrollbarBackTrack;
     }
 }
 
 } // namespace blink