Fix a crash in AXObject::accessibilityIsIgnored() by adding NULL check on axObjectCache().

BUG=368592
TEST=none

Review URL: https://codereview.chromium.org/262663002

git-svn-id: svn://svn.chromium.org/blink/trunk@172987 bbb929c8-8fbe-4397-9dbb-9b2b20218538