Fixed compilation and a crash when PRELOAD_DEBUG is set in ResourceFetcher.cpp

BUG=
R=japhet@chromium.org

Review URL: https://codereview.chromium.org/155093003

git-svn-id: svn://svn.chromium.org/blink/trunk@166534 bbb929c8-8fbe-4397-9dbb-9b2b20218538