Fix navigation timing in ESR 38

As visible in http://test-reports.tbb.torproject.org/reports/r/gk-tbb-nightly-Fedora20-x86_64/ and other test builds with ESR 38 our navigation timing test is failing with something like

responseEnd - '1953' should equal '0'

This seems to be a regression on Mozilla's side we should investigate and get fixed.