@page rule missing from simpleUserAgentStyleSheet.

simpleUserAgentStyleSheet did not have an @page rule like html.css.
The result was flakiness for the following test depending on whether the full
UA styles had been triggered by a previous test or not:

LayoutTests/printing/page-format-data.html

R=ojan
BUG=303324

Review URL: https://codereview.chromium.org/25856002

git-svn-id: svn://svn.chromium.org/blink/trunk@158866 bbb929c8-8fbe-4397-9dbb-9b2b20218538