Unloaded web fonts should not report Resource Timing

ResourceFetcher::requestLoadStarted() should not report Resource Timing
when the resource is not started loading.

BUG=389038
TEST=fast/css/resource-timing-for-unloaded-font.html

Review URL: https://codereview.chromium.org/355973002

git-svn-id: svn://svn.chromium.org/blink/trunk@177386 bbb929c8-8fbe-4397-9dbb-9b2b20218538