refer to WebServiceWorkerCacheError values in c++03 style

Since WebServiceWorkerCacheError values are prefixed with
"WebServiceWorkerCacheError", the c++11 style enum prefix
is a bit redundant.

BUG=392621

Review URL: https://codereview.chromium.org/489473002

git-svn-id: svn://svn.chromium.org/blink/trunk@180548 bbb929c8-8fbe-4397-9dbb-9b2b20218538