Add comments for SlideAnimation::Show/Hide methods

From the header it's not obvious what Show/Hide means in terms of this
class' behaviour. This CL adds clarification as to their effect on
GetCurrentValue().

Change-Id: Ib5ebe34a687c6562442a27172b767b5d0f8fe3a7
Reviewed-on: https://chromium-review.googlesource.com/894862Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533295}

ui/gfx/animation/slide_animation.h

@@ -55,9 +55,11 @@ class ANIMATION_EXPORT SlideAnimation : public LinearAnimation {
   virtual void Reset(double value);
 
   // Begin a showing animation or reverse a hiding animation in progress.
+  // Animates |GetCurrentValue()| towards 1.
   virtual void Show();
 
   // Begin a hiding animation or reverse a showing animation in progress.
+  // Animates |GetCurrentValue()| towards 0.
   virtual void Hide();
 
   // Sets the time a slide will take. Note that this isn't actually