Remove CachedResource::canBeAccessedBy.

It is neither implemented nor called. This patch removes it.

R=marja@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19543017

git-svn-id: svn://svn.chromium.org/blink/trunk@154752 bbb929c8-8fbe-4397-9dbb-9b2b20218538