AppCache error details tests

Follows Blink (crrev.com/164873003) and Chromium (crrev.com/164933002)

BUG=342555

Review URL: https://codereview.chromium.org/217133002

git-svn-id: svn://svn.chromium.org/blink/trunk@170621 bbb929c8-8fbe-4397-9dbb-9b2b20218538