Fix aspect ratio of <video> in fullscreen mode.

For video element, if the ObjectFitPosition flag is not enabled, we use ObjectFitContain by default.

BUG=282484
TEST=<video> aspect ratio in fullscreen mode is correct with or without
ObjectFitPosition flag enabled.

Review URL: https://chromiumcodereview.appspot.com/23496038

git-svn-id: svn://svn.chromium.org/blink/trunk@157927 bbb929c8-8fbe-4397-9dbb-9b2b20218538