Fix sizing of non strech items

Summary:
Fixes the sizing of items so that under most scenarios it calcultes its height by it's content for non exact measurings. This introduces a new useLegacyStretchBehaviour flag on the config to opt out of this change as it is breaking.

See facebook/yoga#505
Closes https://github.com/facebook/yoga/pull/506

Reviewed By: astreet

Differential Revision: D4954016

Pulled By: emilsjolander

fbshipit-source-id: d28bd5d174cd76951fb94df85e3b0cfab7f81ff7

enums.py

@@ -94,7 +94,6 @@ ENUMS = {
     'ExperimentalFeature': [
         # Mimic web flex-basis behavior.
         'WebFlexBasis',
-        'MinFlexFix'
     ],
     'PrintOptions': [
         ('Layout', 1),