Reland "Event Timing: When next paint not needed, assign fallback time"

This reverts commit f88fc95a51d36ee58f903f88c87bf205dd5e0ba7.

Reason for revert: Patch was reverted because of a flaky test.  Test has been fixed, without any changes to original patch.

Original change's description:
> Revert "Event Timing: When next paint not needed, assign fallback time"
>
> This reverts commit 22302c983dd2586924b59b4ddadb310665ba48c2.
>
> Reason for revert: This causes flakiness of external/wpt/event-timing/modal-dialog-interrupt-paint.html, see crbug.com/381457334
>
> Original change's description:
> > Event Timing: When next paint not needed, assign fallback time
> >
> > Now that we can use either presentation time or fallback time as the end
> > time of Event Timings, switch over to using fallback time for cases
> > where the event does not actually measure a next paint.
> >
> > This cleans up some tracing details and simplifies some of the future
> > work that will need to differentiate events that do and dont paint.
> >
> > Bug: 328902994, 361727943, 378647854
> > Change-Id: I96a4d45eac135a93d1ff4907d21632dd04c7e041
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5876692
> > Commit-Queue: Michal Mocny <[email protected]>
> > Reviewed-by: Ian Clelland <[email protected]>
> > Cr-Commit-Position: refs/heads/main@{#1389571}
>
> Bug: 328902994, 361727943, 378647854, 381457334
> Change-Id: I01fee921f06732918423a87cfdc0bde107fc13b6
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6059815
> Commit-Queue: Rubber Stamper <[email protected]>
> Owners-Override: Antonio Sartori <[email protected]>
> Bot-Commit: Rubber Stamper <[email protected]>
> Auto-Submit: Antonio Sartori <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1389729}

Bug: 328902994, 361727943, 378647854, 381457334
Change-Id: Icd129c15890d071b552531fd06d90e13dd9423a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6059284
Auto-Submit: Michal Mocny <[email protected]>
Reviewed-by: Ian Clelland <[email protected]>
Commit-Queue: Ian Clelland <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1389883}

event-timing/modal-dialog-interrupt-paint.html

@@ -16,6 +16,9 @@
   const events = ['click'];
   const showModalDialog = () => {
     alert();
+    // Stay busy for a few more ms after dialog closes, to ensure processingEnd
+    // time is far from startTime + duration
+    mainThreadBusy(16);
   };
   document.getElementById('testButtonId').addEventListener("click", showModalDialog);