Introduce NON_ZERO_DURATION for animation unit tests

ui/compositor/scoped_animation_duration_scale_mode.h

 // Copyright (c) 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.
 
 #ifndef UI_COMPOSITOR_SCOPED_ANIMATION_DURATION_SCALE_MODE_H_
 #define UI_COMPOSITOR_SCOPED_ANIMATION_DURATION_SCALE_MODE_H_
 
 #include "base/basictypes.h"
 #include "ui/compositor/compositor_export.h"
 
 namespace ui {
 
 // Speed up or slow down animations for testing or debugging.
 class COMPOSITOR_EXPORT ScopedAnimationDurationScaleMode {
  public:
   enum DurationScaleMode {
     NORMAL_DURATION,
     FAST_DURATION,
     SLOW_DURATION,
+    // A very short but guaranteed non-zero duration for individual tests that
+    // need to assert things about animations after creating them.
+    NON_ZERO_DURATION,
+    // Animations complete immediately after being created. Used by most tests.
     ZERO_DURATION
   };
 
   explicit ScopedAnimationDurationScaleMode(
       DurationScaleMode scoped_duration_scale_mode)
       : old_duration_scale_mode_(duration_scale_mode_) {
     duration_scale_mode_ = scoped_duration_scale_mode;
   }
 
   ~ScopedAnimationDurationScaleMode() {
     duration_scale_mode_ = old_duration_scale_mode_;
   }
 
   static DurationScaleMode duration_scale_mode() {
     return duration_scale_mode_;
   }
 
  private:
   DurationScaleMode old_duration_scale_mode_;
 
   static DurationScaleMode duration_scale_mode_;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedAnimationDurationScaleMode);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_SCOPED_ANIMATION_DURATION_SCALE_MODE_H