Hide scrollbars when their dimensions are not scrollable.

cc/animation/scrollbar_animation_controller_linear_fade.cc

 // Copyright 2012 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 "cc/animation/scrollbar_animation_controller_linear_fade.h"
 
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
 
@@ skipping 44 matching lines @@
 void ScrollbarAnimationControllerLinearFade::ApplyOpacityToScrollbars(
     float opacity) {
   if (!scroll_layer_->scrollbars())
     return;
 
   LayerImpl::ScrollbarSet* scrollbars = scroll_layer_->scrollbars();
   for (LayerImpl::ScrollbarSet::iterator it = scrollbars->begin();
        it != scrollbars->end();
        ++it) {
     ScrollbarLayerImplBase* scrollbar = *it;
-    if (scrollbar->is_overlay_scrollbar())
-      scrollbar->SetOpacity(opacity);
+    if (scrollbar->is_overlay_scrollbar()) {
+      bool visible = scroll_layer_->user_scrollable(scrollbar->orientation()) &&

[aelias_OOO_until_Jul13, 2014/10/13 18:30:03]

How about moving this logic to a method on ScrollbarLayerImplBase, called
scrollable()?

+                     scrollbar->maximum() > 0;
+      scrollbar->SetOpacity(visible ? opacity : 0);
+    }
   }
 }
 
 }  // namespace cc