Paint flex items atomically, like inline-blocks

As required by http://dev.w3.org/csswg/css-flexbox/#painting

R=ojan@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/23737004

git-svn-id: svn://svn.chromium.org/blink/trunk@157004 bbb929c8-8fbe-4397-9dbb-9b2b20218538