[EventTiming] Ship Event Timing API

Intent to Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/1UQqjz1CR-k

Bug: 543598
Change-Id: I2f878bdfb2e5390754b89ef7fa61f4213e679003
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216652Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773176}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -684,7 +684,7 @@
     },
     {
       name: "EventTiming",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "ExecCommandInJavaScript",

third_party/blink/web_tests/virtual/stable/fast/dom/Window/window-properties-performance-expected.txt

@@ -6,6 +6,13 @@ window.performance.clearMarks [function]
 window.performance.clearMeasures [function]
 window.performance.clearResourceTimings [function]
 window.performance.dispatchEvent [function]
+window.performance.eventCounts [object EventCounts]
+window.performance.eventCounts.entries [function]
+window.performance.eventCounts.forEach [function]
+window.performance.eventCounts.get [function]
+window.performance.eventCounts.has [function]
+window.performance.eventCounts.keys [function]
+window.performance.eventCounts.values [function]
 window.performance.getEntries [function]
 window.performance.getEntriesByName [function]
 window.performance.getEntriesByType [function]

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -1983,6 +1983,17 @@ interface Event
     method stopPropagation
     setter cancelBubble
     setter returnValue
+interface EventCounts
+    attribute @@toStringTag
+    getter size
+    method @@iterator
+    method constructor
+    method entries
+    method forEach
+    method get
+    method has
+    method keys
+    method values
 interface EventSource : EventTarget
     attribute @@toStringTag
     attribute CLOSED
@@ -4977,6 +4988,7 @@ interface PaymentResponse : EventTarget
     setter onpayerdetailchange
 interface Performance : EventTarget
     attribute @@toStringTag
+    getter eventCounts
     getter memory
     getter navigation
     getter onresourcetimingbufferfull
@@ -5021,6 +5033,7 @@ interface PerformanceEventTiming : PerformanceEntry
     getter cancelable
     getter processingEnd
     getter processingStart
+    getter target
     method constructor
     method toJSON
 interface PerformanceLongTaskTiming : PerformanceEntry