Move logic for creating a new window to a separate file.

This logic is currently spread between DOMWindow.cpp and FrameLoader.cpp, in
two different static functions. These function could certainly be cleaned up,
but isolating them seems to be a good start.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19104002

git-svn-id: svn://svn.chromium.org/blink/trunk@154138 bbb929c8-8fbe-4397-9dbb-9b2b20218538