New Test for 'vertical-align has no effect on a flex item'

The spec says that 'vertical-align' has no effect on a flex item.
Currently there are no tests for specifically checking this, so
added a new test case for verifying the same.

Review URL: https://codereview.chromium.org/561193003

git-svn-id: svn://svn.chromium.org/blink/trunk@183308 bbb929c8-8fbe-4397-9dbb-9b2b20218538