Introduce NON_ZERO_DURATION for animation unit tests

ui/compositor/scoped_animation_duration_scale_mode.h

Index: ui/compositor/scoped_animation_duration_scale_mode.h
diff --git a/ui/compositor/scoped_animation_duration_scale_mode.h b/ui/compositor/scoped_animation_duration_scale_mode.h
index a0ac5d32bcca52588498fec3e836bf937a627d13..bd893bc78c67b6d5674384a1160c9de291df30aa 100644
--- a/ui/compositor/scoped_animation_duration_scale_mode.h
+++ b/ui/compositor/scoped_animation_duration_scale_mode.h
@@ -17,6 +17,11 @@ class COMPOSITOR_EXPORT ScopedAnimationDurationScaleMode {
     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.
+    TEST_DURATION,
+    // Animations complete immediately after being created. Used by most unit
+    // test suites.
     ZERO_DURATION
   };