Fixup recent fix to flex basis and put it behind an experimental flag

Summary: D4207106 previously fixed an issue where the computed flex basis was cached in between layout calculations, potentially caching wrong values. After landing we noticed that some product were implicitly relying on this behavior so this diff instead puts that behind a feature flag.

Reviewed By: gkassabli

Differential Revision: D4222910

fbshipit-source-id: d693482441fcc4d37a288e2e3529057a04f60541

CSSLayoutKit/UIView+CSSLayout.m

@@ -16,6 +16,12 @@
 @end
 
 @implementation CSSNodeBridge
+
++ (void)initialize
+{
+  CSSLayoutSetExperimentalFeatureEnabled(CSSExperimentalFeatureWebFlexBasis, true);
+}
+
 - (instancetype)init
 {
   if ([super init]) {