Assert that frame() derefs a non-null pointer.

The Frame references kept by these objects are assumed to be non-null;
assert that it is so.

R=haraken
BUG=340522

Review URL: https://codereview.chromium.org/593583002

git-svn-id: svn://svn.chromium.org/blink/trunk@182394 bbb929c8-8fbe-4397-9dbb-9b2b20218538