Remove setOpenedByDOM calls in CreateWindow.cpp.

They are now redundant with calls in Chromium code, as
of https://codereview.chromium.org/212703005

BUG=357579

Review URL: https://codereview.chromium.org/224173007

git-svn-id: svn://svn.chromium.org/blink/trunk@171881 bbb929c8-8fbe-4397-9dbb-9b2b20218538