Re-add snap-back behavior (Chrome side)

content/renderer/webscrollbarbehavior_impl_mac.mm

+// 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.
+
+#include "content/renderer/webscrollbarbehavior_impl_mac.h"
+
+namespace content {
+
+WebScrollbarBehaviorImpl::WebScrollbarBehaviorImpl()
+    : jump_on_track_click_(false) {
+}
+
+bool WebScrollbarBehaviorImpl::shouldCenterOnThumb(
+      blink::WebScrollbarBehavior::Button mouseButton,
+      bool shiftKeyPressed,
+      bool altKeyPressed) {
+  return (mouseButton == blink::WebScrollbarBehavior::ButtonLeft) &&
+      (jump_on_track_click_ != altKeyPressed);
+}
+
+}  // namespace content