Turn a bunch of ASSERTs into graceful failures when compiling listeners

BUG=456192
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/906193002

git-svn-id: svn://svn.chromium.org/blink/trunk@189796 bbb929c8-8fbe-4397-9dbb-9b2b20218538