Clip scrollbars to box bounds when they don't fit.

third_party/WebKit/Source/core/paint/BoxClipper.h

Index: third_party/WebKit/Source/core/paint/BoxClipper.h
diff --git a/third_party/WebKit/Source/core/paint/BoxClipper.h b/third_party/WebKit/Source/core/paint/BoxClipper.h
index 5f06af8db3f489be5d1a2c5bd984bcfbb9469447..d46ebf0fbc6e901b9f1d9b7099543010ef9b700b 100644
--- a/third_party/WebKit/Source/core/paint/BoxClipper.h
+++ b/third_party/WebKit/Source/core/paint/BoxClipper.h
@@ -14,7 +14,11 @@ namespace blink {
 class LayoutBox;
 struct PaintInfo;
 
-enum ContentsClipBehavior { ForceContentsClip, SkipContentsClipIfPossible };
+enum ContentsClipBehavior {
+    ForceContentsClip,
+    SkipContentsClipIfPossible,
+    ClipForScrollbars
+};
 
 class BoxClipper {
     STACK_ALLOCATED();