[EventTiming] Make PerformanceEventTiming Exposed=Window

This CL makes PerformanceEventTiming IDL consistent with the spec, after
https://github.com/WICG/event-timing/pull/16. Note that EventCount is
not yet implemented.

Bug: 823744
Change-Id: I01779ed82cba0cfea0542ab74cc95b4e7adc8aac
Reviewed-on: https://chromium-review.googlesource.com/c/1363961Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614360}

third_party/blink/renderer/core/timing/performance_event_timing.idl

@@ -4,6 +4,7 @@
 
 // https://github.com/wicg/event-timing
 [
+    Exposed=Window,
     OriginTrialEnabled=EventTiming
 ] interface PerformanceEventTiming : PerformanceEntry {
     readonly attribute DOMHighResTimeStamp processingStart;