Add another test for whitespace handling in flexbox

This test makes sure that we don't create renderers for whitespace flex
items even when they're surrounded by inlines.

We get it right, but I didn't consider this case when I
wrote this test, and I felt it was worth testing.

R=ojan@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/21113003

git-svn-id: svn://svn.chromium.org/blink/trunk@155198 bbb929c8-8fbe-4397-9dbb-9b2b20218538