Assert if we try to access another AX attribute while a layout is pending.

Actual fix for crash is in https://codereview.chromium.org/469803003

BUG=393829

Review URL: https://codereview.chromium.org/465143003

git-svn-id: svn://svn.chromium.org/blink/trunk@180739 bbb929c8-8fbe-4397-9dbb-9b2b20218538