Centering text inside a button set to display flex with justify-content: center is impossible

When we set button display to flex and set justify-content: center
chrome does not render text in center.
This patch removes this bug by setting justify-content property with
value specified in style for the flexbox.

BUG=344733

Review URL: https://codereview.chromium.org/278603002

git-svn-id: svn://svn.chromium.org/blink/trunk@174761 bbb929c8-8fbe-4397-9dbb-9b2b20218538