Service Workers: Improve object equality assertion errors

Use Object.prototype.toString.call() to get a name for the object
being tested, rather than '[object]', to make the error more readable.

BUG=374802
R=asanka@chromium.org

Review URL: https://codereview.chromium.org/667423005

git-svn-id: svn://svn.chromium.org/blink/trunk@184381 bbb929c8-8fbe-4397-9dbb-9b2b20218538