Avoid unnecessary forced layout of flex items when stretching

An old optimization to RenderFlexibleBox layout considered whether
or not an element can stretch in the orthogonal direction to flow
to determine whether or not to force layout. This case is already
handled properly in applyStretchAlignmentToChild, so this was
unnecessarily causing extra layouts. Fixing this speeds up our
performance on the Holiday Lites benchmark:
http://ie.microsoft.com/testdrive/Performance/HolidayLites/

TEST=PerformanceTests/flexbox-with-stretch-layout.html
BUG=331848

Review URL: https://codereview.chromium.org/129633004

git-svn-id: svn://svn.chromium.org/blink/trunk@164728 bbb929c8-8fbe-4397-9dbb-9b2b20218538