[ElementTiming] Set startTime

Bug: 879270, 942033
Change-Id: I2cf90c23c3e1e3c81850bcb40e54e2ae39117dd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719407Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681101}

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

@@ -26,13 +26,15 @@ PerformanceElementTiming* PerformanceElementTiming::Create(
   DCHECK_GE(naturalWidth, 0);
   DCHECK_GE(naturalHeight, 0);
   DCHECK(element);
+  double start_time = render_time != 0.0 ? render_time : load_time;
   return MakeGarbageCollected<PerformanceElementTiming>(
-      name, url, intersection_rect, render_time, load_time, identifier,
-      naturalWidth, naturalHeight, id, element);
+      name, start_time, url, intersection_rect, render_time, load_time,
+      identifier, naturalWidth, naturalHeight, id, element);
 }
 
 PerformanceElementTiming::PerformanceElementTiming(
     const AtomicString& name,
+    DOMHighResTimeStamp start_time,
     const String& url,
     const FloatRect& intersection_rect,
     DOMHighResTimeStamp render_time,
@@ -42,7 +44,7 @@ PerformanceElementTiming::PerformanceElementTiming(
     int naturalHeight,
     const AtomicString& id,
     Element* element)
-    : PerformanceEntry(name, 0, 0),
+    : PerformanceEntry(name, start_time, start_time),
       element_(element),
       intersection_rect_(DOMRectReadOnly::FromFloatRect(intersection_rect)),
       render_time_(render_time),

third_party/blink/renderer/core/timing/performance_element_timing.h

@@ -32,6 +32,7 @@ class CORE_EXPORT PerformanceElementTiming final : public PerformanceEntry {
                                           const AtomicString& id,
                                           Element*);
   PerformanceElementTiming(const AtomicString& name,
+                           DOMHighResTimeStamp start_time,
                            const String& url,
                            const FloatRect& intersection_rect,
                            DOMHighResTimeStamp render_time,

third_party/blink/web_tests/external/wpt/element-timing/resources/element-timing-helpers.js

@@ -4,7 +4,13 @@ function checkElementInternal(entry, expectedUrl, expectedIdentifier, expectedID
   assert_equals(entry.entryType, 'element');
   assert_equals(entry.url, expectedUrl);
   assert_equals(entry.identifier, expectedIdentifier);
-  assert_equals(entry.startTime, 0);
+  if (beforeRender != 0) {
+    // In this case, renderTime is not 0.
+    assert_equals(entry.startTime, entry.renderTime);
+  } else {
+    // In this case, renderTime is 0, so compare to loadTime.
+    assert_equals(entry.startTime, entry.loadTime);
+  }
   assert_equals(entry.duration, 0);
   assert_equals(entry.id, expectedID);
   assert_greater_than_equal(entry.renderTime, beforeRender);