Fix inspector/device-orientation-success.html flakiness

Currently, clearing the device orientation override does not result in the respective event dispatch.

BUG=313708
R=vsevik, timvolodine

Review URL: https://codereview.chromium.org/52733002

git-svn-id: svn://svn.chromium.org/blink/trunk@161162 bbb929c8-8fbe-4397-9dbb-9b2b20218538