Make CachedResource initiator type names static strings.

These are passed from the worker thread to the main thread, so using
thread local storage is no good. Plus, there's no reason to have
duplicate strings for each thread.

BUG=237801

Review URL: https://chromiumcodereview.appspot.com/18490004

git-svn-id: svn://svn.chromium.org/blink/trunk@154139 bbb929c8-8fbe-4397-9dbb-9b2b20218538