Convert several LocalDOMWindow accessors to return by pointer.

Previously, there was a codebase cleanup to convert things that could
never be null to return by reference instead. However, many Window
attributes are not accessible cross-origin so a RemoteDOMWindow
implementation would have to create a placeholder object to return.
Instead, these accessors have been reverted to return by pointer, so
attempts to access these properties on RemoteDOMWindows will crash
rather than appear to succeed.

BUG=425623
R=japhet@chromium.org
TBR=mkwst@chromium.org

Review URL: https://codereview.chromium.org/679333002

git-svn-id: svn://svn.chromium.org/blink/trunk@184488 bbb929c8-8fbe-4397-9dbb-9b2b20218538