Convert enum to enum class for MaximumFlexSizeRule

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

Bug: 940736
Change-Id: I661e67465a1f38e78b336f93df97cb09f1d36f7a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735447Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Cr-Commit-Position: refs/heads/master@{#684191}

ui/views/layout/flex_layout_types.h

@@ -57,7 +57,7 @@ enum class MinimumFlexSizeRule {
 
 // Describes a simple rule for how a child view should grow in a layout when
 // there is extra size avaialble for that view to occupy.
-enum MaximumFlexSizeRule {
+enum class MaximumFlexSizeRule {
   kPreferred,  // Don't resize above preferred size.
   kUnbounded   // Allow resize to arbitrary size.
 };