Convert enum to enum class for views::LayoutFadeType

Use enum class instead of enum for views::LayoutFadeType.
enum class is more type safety.

Bug: 940736
Change-Id: I9c026d131835d77d212e4084bc84cb703a0638a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2120837
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753529}

ui/views/layout/animating_layout_manager.cc

@@ -43,7 +43,7 @@ ChildLayout* FindChildViewInLayout(ProposedLayout* layout, const View* view) {
 }
 
 // Describes the type of fade, used by LayoutFadeInfo (see below).
-enum LayoutFadeType {
+enum class LayoutFadeType {
   // This view is fading in as part of the current animation.
   kFadingIn,
   // This view is fading out as part of the current animation.