Animate IOS Share extension button.

ios/chrome/share_extension/ui_util.mm

 // Copyright 2016 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.
 
 #import <UIKit/UIKit.h>
 
 #include "ios/chrome/share_extension/ui_util.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 namespace ui_util {
 
+const CGFloat kAnimationDuration = 0.3;
+
 CGFloat AlignValueToPixel(CGFloat value) {
   CGFloat scale = [[UIScreen mainScreen] scale];
   return floor(value * scale) / scale;
 }
 
 void ConstrainAllSidesOfViewToView(UIView* container, UIView* filler) {
   [NSLayoutConstraint activateConstraints:@[
     [filler.leadingAnchor constraintEqualToAnchor:container.leadingAnchor],
     [filler.trailingAnchor constraintEqualToAnchor:container.trailingAnchor],
     [filler.topAnchor constraintEqualToAnchor:container.topAnchor],
     [filler.bottomAnchor constraintEqualToAnchor:container.bottomAnchor],
   ]];
 }
 
 }  // namespace ui_util