Combine checks inside Element::createShadowRoot and add comments

We can combine the runtime flag check with the if statement inside
createShadowRoot. This patch also adds some comments explaining
what the check is about.

Review URL: https://codereview.chromium.org/141703015

git-svn-id: svn://svn.chromium.org/blink/trunk@165956 bbb929c8-8fbe-4397-9dbb-9b2b20218538