Make ScrollbarThemeAura selectively invalidate scrollbar parts.

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 2066 matching lines @@
         m_pendingScrollDelta += scrollDelta;
 
     clearScrollAnchor();
     updateLayersAndCompositingAfterScrollIfNeeded();
     scrollPositionChanged();
     frame().loader().client()->didChangeScrollOffset();
 }
 
 void FrameView::invalidatePaintForTickmarks()
 {
-    if (Scrollbar* scrollbar = verticalScrollbar()) {
-        scrollbar->setTrackNeedsRepaint(true);
-        setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
-    }
+    if (Scrollbar* scrollbar = verticalScrollbar())
+        scrollbar->setNeedsPaintInvalidation(static_cast<ScrollbarPart>(~ThumbPart));
 }
 
 void FrameView::getTickmarks(Vector<IntRect>& tickmarks) const
 {
     if (!m_tickmarks.isEmpty())
         tickmarks = m_tickmarks;
     else
         tickmarks = frame().document()->markers().renderedRectsForMarkers(DocumentMarker::TextMatch);
 }
 
@@ skipping 1918 matching lines @@
     return m_hiddenForThrottling && m_crossOriginForThrottling;
 }
 
 LayoutBox& FrameView::boxForScrollControlPaintInvalidation() const
 {
     ASSERT(layoutView());
     return *layoutView();
 }
 
 } // namespace blink