Fix smooth scroll overshooting when mouse held down in scrollbar track.

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

 /*
  * Copyright (C) 2011 Google 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
@@ skipping 55 matching lines @@
 int ScrollbarThemeOverlay::scrollbarMargin() const
 {
     return m_scrollbarMargin;
 }
 
 bool ScrollbarThemeOverlay::usesOverlayScrollbars() const
 {
     return true;
 }
 
-int ScrollbarThemeOverlay::thumbPosition(const ScrollbarThemeClient& scrollbar)
+int ScrollbarThemeOverlay::thumbPosition(const ScrollbarThemeClient& scrollbar, float scrollPosition)
 {
     if (!scrollbar.totalSize())
         return 0;
 
     int trackLen = trackLength(scrollbar);
-    float proportion = static_cast<float>(scrollbar.currentPos()) / scrollbar.totalSize();
+    float proportion = static_cast<float>(scrollPosition) / scrollbar.totalSize();
     return round(proportion * trackLen);
 }
 
 int ScrollbarThemeOverlay::thumbLength(const ScrollbarThemeClient& scrollbar)
 {
     int trackLen = trackLength(scrollbar);
 
     if (!scrollbar.totalSize())
         return trackLen;
 
@@ skipping 78 matching lines @@
     return ScrollbarTheme::hitTest(scrollbar, position);
 }
 
 ScrollbarThemeOverlay& ScrollbarThemeOverlay::mobileTheme()
 {
     DEFINE_STATIC_LOCAL(ScrollbarThemeOverlay, theme, (3, 3, ScrollbarThemeOverlay::DisallowHitTest, Color(128, 128, 128, 128)));
     return theme;
 }
 
 } // namespace blink