ContainerTiming: support PaintTimingMixin

Expose paintTime and presentationTime in the container timing
entries. The implementation is easy as we were obtaining already
accurate PaintTimingInfo (providing this information), so it was
just setting the value on the entry creation.

Explainer: https://github.com/bloomberg/container-timing/blob/main/README.md
Chromestatus: https://chromestatus.com/feature/5110962817073152
I2P: https://groups.google.com/a/chromium.org/g/blink-dev/c/J-WxY0w7bNk/m/VkqnomK-CAAJ

Ticket for supporting the mixin in the specification:
bloomberg/container-timing#25

Bug: 378827535, 382422286
Change-Id: I98fd8b6821ddff2aeda03e81915a739a88b6d098
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7150902
Reviewed-by: Michal Mocny <[email protected]>
Commit-Queue: José Dapena Paz <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1553483}

Author: jdapena

container-timing/resources/container-timing-helpers.js

@@ -15,6 +15,14 @@ function checkContainerEntry(entry, identifier, last_element_id, beforeRender) {
       'startTime greater than beforeRender');
   assert_greater_than_equal(
       performance.now(), entry.startTime, 'startTime bound by now()')
+
+  // PaintTimingMixin
+  if ("presentationTime" in entry && entry.presentationTime !== null) {
+    assert_greater_than(entry.presentationTime, entry.paintTime);
+    assert_equals(entry.presentationTime, entry.startTime);
+  } else {
+    assert_equals(entry.startTime, entry.paintTime);
+  }
 }
 
 function checkContainerSize(entry, size) {