Re-enable screen_orientation/ tests following mock fixes.

The screen orientation mock was broken and some LayoutTests had to be
disabled. Now that https://codereview.chromium.org/339913002 has landed,
we the disabled tests should be working again.

BUG=389047,388748

Review URL: https://codereview.chromium.org/360493003

git-svn-id: svn://svn.chromium.org/blink/trunk@177169 bbb929c8-8fbe-4397-9dbb-9b2b20218538