Service Worker: Remove redundant assertions from Cache.put() test case

Two of the Cache.put() tests were failing because of crbug.com/424629;
adjust the assertions so that one test fails (as it should) to track
that bug, but relax the assertions in another to verify just the
intended behavior (which passes).

BUG=374802
R=asanka@chromium.org

Review URL: https://codereview.chromium.org/675783004

git-svn-id: svn://svn.chromium.org/blink/trunk@184320 bbb929c8-8fbe-4397-9dbb-9b2b20218538