Use the non-deprecated way to set the button labels

Use label="" instead of the textContent

R=ojan@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/469423005

git-svn-id: svn://svn.chromium.org/blink/trunk@180312 bbb929c8-8fbe-4397-9dbb-9b2b20218538