Test non-crashing updating of worker scope event handlers.

Add a pair of tests to verify that repeatedly updating attribute event
handlers on a worker global scope doesn't fall over.

R=jochen@chromium.org,dslomov@chromium.org
BUG=239669

Review URL: https://codereview.chromium.org/140723002

git-svn-id: svn://svn.chromium.org/blink/trunk@165439 bbb929c8-8fbe-4397-9dbb-9b2b20218538