Metrics changelog: Add a section on testing for CLS fix in latest canary.

Change-Id: I84ce57f72aafe561e677def149316b16e3d37f8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507258Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822230}

docs/speed/metrics_changelog/2020_10_cls.md

@@ -58,6 +58,19 @@ Sites with absolutely positioned content with negative offsets outside the
 viewport will see large regressions in their Cumulative Layout Shift scores
 until the fix rolls out to stable.
 
+### My CLS score regressed in M86. Is that a result of the above-mentioned issue?
+
+You can test the fix on [Chrome Canary](https://www.google.com/chrome/canary/) version
+88.0.4307.0 or later. You can navigate to `chrome://version` (by typing that in
+the URL bar) to verify your browser's version. Either the
+[JavaScript implementation](https://web.dev/cls/#measure-cls-in-javascript) or the
+[Chrome Extension](https://chrome.google.com/webstore/detail/web-vitals/ahfhijdlegdabablpippeagghigmibma?hl=en)
+can be used to check the Cumulative Layout Shift score on both canary and stable
+channels.
+
+If your CLS score returns to expected levels with the fix, there is no need to
+make changes to your site to address the metric changes in Chrome 86.
+
 ## When were users affected?
 
 Most users were updated to Chrome 86 the week of October 19, 2020.