Revert "[EventTiming] Always buffer firstInput"

This reverts commit b6e2cb9b364649724a1577b0c842074e12b82f27.

Reason for revert: Causing WebKitLayout tests failure on linux:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Tests%20%28dbg%29%281%29/76109
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Tests%20%28dbg%29%281%29/76108

Original change's description:
> [EventTiming] Always buffer firstInput
> 
> Before, we'd buffer the firstInput entry only if it happened before
> onload. This CL forces firstInput to always be buffered. This means
> EventTiming code is called more in some tests, so the event hardware
> timestamp is no longer guaranteed to be nonzero, and a couple of xr
> tests require double RAF to avoid competing with the swap promises from
> Event Timing.
> 
> Bug: 841224, 843184
> 
> Change-Id: I942b934c387798c100da4ecfff52affb66ab94e8
> Reviewed-on: https://chromium-review.googlesource.com/c/1355880
> Reviewed-by: Timothy Dresser <tdresser@chromium.org>
> Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#612703}

TBR=tdresser@chromium.org,npm@chromium.org

Change-Id: I4b2456f4b9f44798b3157bea960d84f72a50a8bb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 841224, 843184
Reviewed-on: https://chromium-review.googlesource.com/c/1357404
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612790}
8 files changed