Remove ResourceFetcher::cachedResource(String) method

It only has a single caller, and in a future where URLs are normally
completed per-TreeScope (see attached bug), I'm trying to push
URL-completion as early as possible.

R=japhet@chromium.org
BUG=311682

Review URL: https://codereview.chromium.org/129553002

git-svn-id: svn://svn.chromium.org/blink/trunk@164727 bbb929c8-8fbe-4397-9dbb-9b2b20218538