WebDocument::accessibilityObject should check axObjectCache

Document::axObjectCache() may return NULL, so it should be checked and
handled. This was reported by Huawei Technologies based on a stack trace
of an Android crash.

BUG=None

Review URL: https://codereview.chromium.org/563823002

git-svn-id: svn://svn.chromium.org/blink/trunk@182439 bbb929c8-8fbe-4397-9dbb-9b2b20218538