Implement standard interface for toggling experimental features

Summary: We want to start experimenting with changes to the library in a more controlled and easier to measure manner. This diff adds the basics of an experiment layer to csslayout.

Reviewed By: gkassabli

Differential Revision: D4174260

fbshipit-source-id: ad3667183810c02833fba9a1276f89286e848fcd

CSSLayout/CSSLayout.c

@@ -2512,3 +2512,13 @@ void CSSLog(CSSLogLevel level, const char *format, ...) {
   gLogger(level, format, args);
   va_end(args);
 }
+
+static bool experimentalFeatures[CSSExperimentalFeatureCount];
+
+void CSSLayoutSetExperimentalFeatureEnabled(CSSExperimentalFeature feature, bool enabled) {
+  experimentalFeatures[feature] = enabled;
+}
+
+bool CSSLayoutIsExperimentalFeatureEnabled(CSSExperimentalFeature feature) {
+  return experimentalFeatures[feature];
+}