Fullscreen videos shouldn't be affected by CSS left/top properties

This fixes a regression introduced in
https://chromiumcodereview.appspot.com/22454003 which added position:relative
to fullscreen videos to force stacking context. It was previously statically
positioned to ignore top/left properties. This CL adds left:0 and top:0 to
override any user-supplied top/left properties.

BUG=315540

Review URL: https://codereview.chromium.org/149773007

git-svn-id: svn://svn.chromium.org/blink/trunk@167073 bbb929c8-8fbe-4397-9dbb-9b2b20218538