Re-add snap-back behavior (Blink side)

public/platform/WebScrollbarBehavior.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef WebScrollbarBehavior_h
+#define WebScrollbarBehavior_h
+
+namespace blink {
+
+struct WebPoint;
+struct WebRect;
+
+class WebScrollbarBehavior {
+public:
+    // These match the values for WebCore::MouseButton.

[Peter Kasting, 2014/02/25 00:01:58]

Let me know if I need a comment over on that enum (I assume not, since
WebInputEvent.h has this same sort of enum) or some kind of compile-time
equality check of the values.

[abarth-chromium, 2014/02/25 07:18:38]

Checkout out AssertMatchingEnums.cpp.  We usually just have compile asserts
instead of comments.

+    enum Button {
+        ButtonNone = -1,
+        ButtonLeft,
+        ButtonMiddle,
+        ButtonRight
+    };
+    virtual bool shouldCenterOnThumb(Button, bool shiftKeyPressed, bool altKeyPressed) { return false; }
+    virtual bool shouldSnapBackToDragOrigin(const WebPoint& eventPoint, const WebRect& scrollbarRect, bool isHorizontal) { return false; }
+};
+
+} // namespace blink
+
+#endif