Move plugin placeholder style to CSS, and allow it to bypass main world CSP.

<style> elements inside UA shadow roots are permitted to load inline style,
even if the host document CSP disallows this, since the style is supplied by
the user agent.

BUG=364716

Review URL: https://codereview.chromium.org/516273002

git-svn-id: svn://svn.chromium.org/blink/trunk@184015 bbb929c8-8fbe-4397-9dbb-9b2b20218538