Bug 1046645 - Mark moz-page-thumb:// as local resources to prevent mixed content warnings f=Mardak r=gavin a=lmandel

Mixed content warnings on valid https pages may sometimes occur when a thumbnail on about:newtab is refreshed while we're already transitioning to the new web page. The "insecure" request is counted as a subresource request towards the newly started load. If the new target is a https page this may result in a mixed content warning because nsSecureBrowserUI is dumb and can't tell the requests of the current and previous page loads apart.