Enable null GeolocationInspectorAgent for out-of-process iframes.

We are not yet able to use inspector agents when the top-level frame is
in another process. This patch prevents a crash in that scenario.

BUG=357747

Review URL: https://codereview.chromium.org/422943002

git-svn-id: svn://svn.chromium.org/blink/trunk@179052 bbb929c8-8fbe-4397-9dbb-9b2b20218538