bindings: Supports serializer for PerformanceTiming.

LayoutTests/fast/dom/webtiming-document-open-expected.txt

 This test verifies that the NavigationTimings don't change after a document.open().
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 PASS timing.connectEnd is originalTiming.connectEnd
 PASS timing.connectStart is originalTiming.connectStart
 PASS timing.domComplete is originalTiming.domComplete
 PASS timing.domContentLoadedEventEnd is originalTiming.domContentLoadedEventEnd
 PASS timing.domContentLoadedEventStart is originalTiming.domContentLoadedEventStart
 PASS timing.domInteractive is originalTiming.domInteractive
 PASS timing.domLoading is originalTiming.domLoading
 PASS timing.domainLookupEnd is originalTiming.domainLookupEnd
 PASS timing.domainLookupStart is originalTiming.domainLookupStart
 PASS timing.fetchStart is originalTiming.fetchStart
 PASS timing.loadEventEnd is originalTiming.loadEventEnd
 PASS timing.loadEventStart is originalTiming.loadEventStart
 PASS timing.navigationStart is originalTiming.navigationStart
 PASS timing.redirectEnd is originalTiming.redirectEnd
 PASS timing.redirectStart is originalTiming.redirectStart
 PASS timing.requestStart is originalTiming.requestStart
 PASS timing.responseEnd is originalTiming.responseEnd
 PASS timing.responseStart is originalTiming.responseStart
 PASS timing.secureConnectionStart is originalTiming.secureConnectionStart
+PASS timing.toJSON is originalTiming.toJSON
 PASS timing.unloadEventEnd is originalTiming.unloadEventEnd
 PASS timing.unloadEventStart is originalTiming.unloadEventStart
 PASS successfullyParsed is true
 
 TEST COMPLETE