DevTools: [Documentation] fix css style for renderer. 
Code border became light gray; indents appeared.

BUG=391593

Review URL: https://codereview.chromium.org/551463003

git-svn-id: svn://svn.chromium.org/blink/trunk@181886 bbb929c8-8fbe-4397-9dbb-9b2b20218538