Use [ImplementedAs=domWindow] for Document.defaultView()

Use [ImplementedAs=domWindow] for Document.defaultView() to avoid having 2
implementation methods that return the same thing. This makes our
implementation more consistent.

R=haraken
BUG=

Review URL: https://codereview.chromium.org/27183013

git-svn-id: svn://svn.chromium.org/blink/trunk@159715 bbb929c8-8fbe-4397-9dbb-9b2b20218538