Summary:
This fixes shrinking of elements which are in a non stretch alignment, but keeps the legacy stretch behaviour in place. Additionally this adds a testcase for ```useLegacyStretchBehaviour```
Fixes#633
Closes https://github.com/facebook/yoga/pull/635
Differential Revision: D6408037
Pulled By: emilsjolander
fbshipit-source-id: 377ab0308dd3a75a148a0af31ab5eb3ffb5b5d83