Scale scrollbar in use-zoom-for-dsf mode

third_party/WebKit/Source/platform/HostWindow.h

Index: third_party/WebKit/Source/platform/HostWindow.h
diff --git a/third_party/WebKit/Source/platform/HostWindow.h b/third_party/WebKit/Source/platform/HostWindow.h
index 7dbe72b146bb0ea4819723e0c9443b9237bdf1c1..0f7f4017d0c7290e4724b239ffacfd7c451a1af7 100644
--- a/third_party/WebKit/Source/platform/HostWindow.h
+++ b/third_party/WebKit/Source/platform/HostWindow.h
@@ -47,9 +47,12 @@ public:
     // Requests the host invalidate the contents.
     virtual void invalidateRect(const IntRect& updateRect) = 0;
 
-    // Converts from the window coordinates to screen coordinates.
+    // Converts the rect from the viewport coordinates to screen coordinates.
     virtual IntRect viewportToScreen(const IntRect&) const = 0;
 
+    // Converts the length from the screen coordinates to the viewport coordinates.
+    virtual float screenToViewport(float length) const = 0;
+
     virtual void scheduleAnimation(Widget*) = 0;
 };