Test steps:
1. Create a XHR to get serverTiming.sjs.
2. Add Server-Timing headers in serverTiming.sjs.
3. Check if the value from PerformanceResourceTiming is correct.
MozReview-Commit-ID: KOQhoFQv4fy
--HG--
extra : rebase_source : a0f5bde872ca9e066764d90ab80d7848988f37a8
