URLBlocker: don't block things if our main URL is at that domain

This allows visiting a twitter.com URL as the main page in the tab
to not block twitter subrequests since the expected behavior of
going to that URL is that it's not blocked.
URLInterceptor: fix off-by-one in user-agent rebuilding
cleanup origin URL detection and progress bar stuff

WebViewTab progress was getting clobbered among other tabs, so a
background tab loading something would show a progress bar when it
wasn't the current tab.