Fetcher: Disable memory caching when Service Worker handles a request

MemoryCache can cache 1 resouce for 1 URL, but ServiceWorker can serve
arbitrary resources for 1 URL and pollute a memory cache entry.
As a stopgap solution, this change removes a cached resouce to be
controlled by Service Worker before requesting.

BUG=388375
TEST=manual (access a same resource from controlled/non-controlled documents on a same tab)
TEST=run_webkit_tests.py http/tests/serviceworker/chromium/memory-cache.html

Review URL: https://codereview.chromium.org/632213002

git-svn-id: svn://svn.chromium.org/blink/trunk@183534 bbb929c8-8fbe-4397-9dbb-9b2b20218538