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

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

Index: third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
index 4ba3e53da101f32a45c2c2a4af026516f7da1673..e33d72715be566b361d3220b8026f4dfbb7532ab 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollbarTheme.cpp
@@ -207,14 +207,14 @@ bool ScrollbarTheme::shouldSnapBackToDragOrigin(const ScrollbarThemeClient& scro
     return Platform::current()->scrollbarBehavior()->shouldSnapBackToDragOrigin(mousePosition, trackRect(scrollbar), scrollbar.orientation() == HorizontalScrollbar);
 }
 
-int ScrollbarTheme::thumbPosition(const ScrollbarThemeClient& scrollbar)
+int ScrollbarTheme::thumbPosition(const ScrollbarThemeClient& scrollbar, float scrollPosition)
 {
     if (scrollbar.enabled()) {
         float size = scrollbar.totalSize() - scrollbar.visibleSize();
         // Avoid doing a floating point divide by zero and return 1 when usedTotalSize == visibleSize.
         if (!size)
             return 0;
-        float pos = std::max(0.0f, scrollbar.currentPos()) * (trackLength(scrollbar) - thumbLength(scrollbar)) / size;
+        float pos = std::max(0.0f, scrollPosition) * (trackLength(scrollbar) - thumbLength(scrollbar)) / size;
         return (pos < 1 && pos > 0) ? 1 : pos;
     }
     return 0;