Don't issue CSP warnings for preloads

It turns out that only two tests actually hit this
removing this removes two redundant warnings from
those tests.

I'm considering sending all tokens through the preloader
which will increase the number of these double-logging
events, so removing this double-logging before doing that.

BUG=309862

Review URL: https://codereview.chromium.org/248273002

git-svn-id: svn://svn.chromium.org/blink/trunk@172324 bbb929c8-8fbe-4397-9dbb-9b2b20218538