trunk-tracking: update to r3485

* Adds IPRO tests that we should have copied from apache_system_test.sh back
  when we first added IPRO.
  * The "IPRO-optimized resources should have fixed size, not chunked" test
    fails.  The problem is as it says: IPRO optimized resources are being sent
    out with `Transfer-Encoding: Chunked` instead of `Content-Length: ...`.  How
    do we fix this?
* install/system_test.sh moved to net/instaweb/automatic/system_test.sh
* minor method signature changes
* url.is_valid() is gone, replaced by .IsWebValid()
* I pushed Otto's #464 upstream, and now we need to make some IPRO changes.
* beacons have nonces now, so we need to update tests.
Merge pull request #505 from pagespeed/jefftk-update-trunk-tracking

trunk-tracking: update to r3485