Fix a crash caused by calling detached navigator APIs.

Calling a navigator method leads to a crash when:
 - The navigator is associated with an iframe, 
 - The iframe has some contents and
 - The iframe is detached.

This CL fixes the crash.

BUG=358800
R=abarth

Review URL: https://codereview.chromium.org/234583004

git-svn-id: svn://svn.chromium.org/blink/trunk@171451 bbb929c8-8fbe-4397-9dbb-9b2b20218538