Move some SubstituteData loading code out of DocmentLoader::responseReceived

There's no good reason it can't live in handleSubstituteDataLoadNow

BUG=none

Review URL: https://chromiumcodereview.appspot.com/21046007

git-svn-id: svn://svn.chromium.org/blink/trunk@155110 bbb929c8-8fbe-4397-9dbb-9b2b20218538