Handle failure to initialize MockPagePopup

The content frame for the mock page popup might be gone when we try to load.

BUG=373668

Review URL: https://codereview.chromium.org/295623002

git-svn-id: svn://svn.chromium.org/blink/trunk@174341 bbb929c8-8fbe-4397-9dbb-9b2b20218538