[EventTiming] Exclude target from toJSON

This CL removes the target from the return value of toJSON from a
PerformanceEventTiming entry, and modifies the WPT accordignly.
Context: https://github.com/WICG/event-timing/issues/97

Change-Id: I2a2f27f845f297157ca9f5ae561a9730dc242d9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636058
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844762}

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

@@ -85,7 +85,6 @@ void PerformanceEventTiming::BuildJSONValue(V8ObjectBuilder& builder) const {
   builder.AddNumber("processingStart", processingStart());
   builder.AddNumber("processingEnd", processingEnd());
   builder.AddBoolean("cancelable", cancelable_);
-  builder.Add("target", target());
 }
 
 void PerformanceEventTiming::Trace(Visitor* visitor) const {

third_party/blink/web_tests/external/wpt/event-timing/toJSON.html

@@ -27,12 +27,12 @@
           'processingStart',
           'processingEnd',
           'cancelable',
-          'target',
         ];
         for (const key of keys) {
           assert_equals(json[key], entry[key],
             'PerformanceEventTiming ${key} entry does not match its toJSON value');
         }
+        assert_equals(json['target'], undefined, 'toJSON should not include target');
       })
     );
     observer.observe({type: 'event'});