Convert enum to enum class for BoundsAnimationMode

Use enum class instead of enum for BoundsAnimationMode.
enum class is more type safety.

Bug: 940736
Change-Id: Idc09b7b7f3beec8b179a989e1f5a7e02d7d4734f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2400581
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805291}

ui/views/layout/animating_layout_manager.h

@@ -74,7 +74,7 @@ class VIEWS_EXPORT AnimatingLayoutManager : public LayoutManagerBase {
 
   // Describes if and how the bounds of the host view can be animated as part of
   // layout animations, if the preferred size of the layout changes.
-  enum BoundsAnimationMode {
+  enum class BoundsAnimationMode {
     // Default behavior: the host view will always take the space given to it by
     // its parent view and child views will animate within those bounds. Useful
     // for cases where the layout is in a fixed-size container or dialog, but