Rename CachedResourceLoader to ResourceFetcher

This is a first part of the series of name under core/loader.
This renames CachedResourceRequest to FetchRequest as well.

BUG=263277
TEST=none
R=japhet@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/19617008

git-svn-id: svn://svn.chromium.org/blink/trunk@154823 bbb929c8-8fbe-4397-9dbb-9b2b20218538