Add a protector to DocumentLoader::handleSubstituteDataLoadNow()

responseReceived()'s protector now longer does everything it needs, post r155110

BUG=266364
TEST=fast/loader/open-in-srcdoc-unload.html (under asan)

Review URL: https://chromiumcodereview.appspot.com/21369004

git-svn-id: svn://svn.chromium.org/blink/trunk@155267 bbb929c8-8fbe-4397-9dbb-9b2b20218538