Rename DOMWindow to LocalDOMWindow

For out-of-process iframes, we require an alternate limited
functionality DOMWindow to be bound to JavaScript window objects.

Renaming DOMWindow to LocalDOMWindow is allows us to later create a
DOMWindow interface class, that is implemented by LocalDOMWindow and
RemoteDOMWindow.

Discussion:
https://groups.google.com/a/chromium.org/d/msg/site-isolation-dev/BC0vAaxCSuM/ChoG4e_lXEYJ

BUG=346764

Review URL: https://codereview.chromium.org/334283004

git-svn-id: svn://svn.chromium.org/blink/trunk@176535 bbb929c8-8fbe-4397-9dbb-9b2b20218538