Fix incorrect assumption about children's flexBasis

The code to cache preferredMainAxisContentExtentForChild when possible
and re-use to avoid forcing layout was broken in the case that a child's
flex basis changed to cause it to be dependent on layout, triggering
an assert in debug. Just pushing it down the normal path fixes the
issue.

BUG=366185

Review URL: https://codereview.chromium.org/655053002

git-svn-id: svn://svn.chromium.org/blink/trunk@183853 bbb929c8-8fbe-4397-9dbb-9b2b20218538