Add default case to blink::WebAXEvent switch.

This is necessary so we can add new enum values in Blink later
without breaking the Chromium compile.

BUG=none
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/174783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252882 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252882