Add ViewConfiguration and make our scrollbar size match Android

sky/framework/view-configuration.dart

Index: sky/framework/view-configuration.dart
diff --git a/sky/framework/view-configuration.dart b/sky/framework/view-configuration.dart
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index 0000000000000000000000000000000000000000..984c9cab615351cfcd9343d30750d6a4b59f4ded
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+++ b/sky/framework/view-configuration.dart
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+// Copyright 2015 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.
+
+// Modeled after Android's ViewConfiguration:
+// https://github.com/android/platform_frameworks_base/blob/master/core/java/android/view/ViewConfiguration.java
+
+const int kScrollbarSize = 10;
+const int kScrollbarFadeDelay = 300;
+const int kScrollbarFadeDuration = 250;