Hide scrollbars when their dimensions are not scrollable.

cc/animation/scrollbar_animation_controller_thinning.cc

 // Copyright 2013 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_thinning.h"
 
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
 
@@ skipping 127 matching lines @@
     float opacity, float thumb_thickness_scale) {
   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(
-          AdjustScale(opacity, scrollbar->opacity(), opacity_change_));
+      float effectiveOpacity =
+          scrollbar->CanScrollOrientation()
+              ? AdjustScale(opacity, scrollbar->opacity(), opacity_change_)
+              : 0;
+
+      scrollbar->SetOpacity(effectiveOpacity);
       scrollbar->SetThumbThicknessScaleFactor(
           AdjustScale(thumb_thickness_scale,
                       scrollbar->thumb_thickness_scale_factor(),
                       thickness_change_));
     }
   }
 }
 
 }  // namespace cc