Update metrics changelog for M85 and M86.

Also include a change missed in M84.

Change-Id: I0091c73b5d56fee3d326412a367a1cb85cdbd865
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2373218
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801391}

docs/speed/metrics_changelog/2020_06_cls.md

+# Cumulative Layout Shift Changes in M85
+
+## Changes in Chrome 85
+Prior to Chrome 85, there was a [bug](https://bugs.chromium.org/p/chromium/issues/detail?id=1088311)
+in Cumulative Layout Shift on pages with video elements. Hovering over the video
+element so that the thumb slider was visible would result in layout shifts when
+it moved. The bug was fixed in Chrome 85. The source code of the change can be
+seen [here](https://chromium-review.googlesource.com/c/chromium/src/+/2233310).
+
+## How does this affect a site's metrics?
+
+This change only affects metrics for a very small amount of sites. Desktop sites
+with video elements that users hover their mouse over for an extended period of
+time will have lower CLS values starting in Chrome 85.
+
+We do not see an impact from this change in our overall metrics, so we believe
+the effect on most sites will be minimal.
+
+## When were users affected?
+
+Most users were updated to Chrome 85 the week of August 24, 2020.

docs/speed/metrics_changelog/2020_06_fcp.md

+# First Contentful Paint Changes in M84
+
+## Changes in Chrome 84
+Starting in Chrome 84, content with opacity:0 is no longer counted as the
+first contentful paint. This brings behavior in line with the
+[specification](https://www.w3.org/TR/paint-timing/).
+The source code of the change can be seen
+[here](https://chromium-review.googlesource.com/c/chromium/src/+/2145134).
+
+## How does this affect a site's metrics?
+
+This change affects sites whose content is all opacity:0 during the first paint.
+After this change, the first contentful paint will be reported the next time
+visible content paints.
+
+We do not see an impact from this change in our overall metrics, so we believe
+the effect on most sites will be minimal.
+
+## When were users affected?
+
+Most users were updated to Chrome 84 the week of July 13, 2020.

docs/speed/metrics_changelog/2020_07_fcp.md

+# First Contentful Paint Changes in M86
+
+## Changes in Chrome 86
+In Chrome 86, some small changes were made to First Contentful Paint to bring
+its implementation in line with the [specification](https://www.w3.org/TR/paint-timing/).
+
+The changes are:
+ * Video elements now trigger FCP when painted. [Source code for this change](https://chromium-review.googlesource.com/c/chromium/src/+/2276244).
+ * Only SVG elements with content now trigger FCP. Previously empty SVG paints triggered SVG. [Source code for this change](https://chromium-review.googlesource.com/c/chromium/src/+/2285532).
+ * WebGL2 canvases now trigger FCP when painted. [Source code for this change](https://chromium-review.googlesource.com/c/chromium/src/+/2348694).
+
+## How does this affect a site's metrics?
+
+This change affects a small set of sites:
+ * Sites with a first contentful paint which is only a video element without a `poster` attribute will have an earlier FCP time.
+ * Sites with a first contentful paint which is only an empty SVG element will have a later FCP time.
+ * Sites with a first contentful paint which is only a WebGL canvas will have an earlier FCP time.
+
+We do not see an impact from this change in our overall metrics, so we believe
+the effect on most sites will be minimal.
+
+## When were users affected?
+
+Most users were updated to Chrome 86 the week of October 5, 2020.

docs/speed/metrics_changelog/2020_08_lcp.md

+# Largest Contentful Paint Changes in M86
+
+## Changes in Chrome 86
+Prior to Chrome 86, there was a [bug](https://bugs.chromium.org/p/chromium/issues/detail?id=1092473)
+in Largest Contentful Paint on some pages where the largest contentful element
+initially has opacity:0. The bug was fixed in Chrome 86. The source code of the
+change can be seen [here](https://chromium-review.googlesource.com/c/chromium/src/+/2316788).
+
+## How does this affect a site's metrics?
+
+This change only affects metrics for a very small amount of sites. Generally
+sites whose largest contentful elements are opacity:0 initially are using A/B
+testing frameworks which initially clear the page before displaying the correct
+content. Sites using this technique will now see a longer LCP, reflecting the
+time until the largest contentful paint is visible to the user, instead of the
+time it is loaded in the DOM but invisible.
+
+We do not see an impact from this change in our overall metrics, so we believe
+the effect on most sites will be minimal.
+
+## When were users affected?
+
+Most users were updated to Chrome 86 the week of October 5, 2020.

docs/speed/metrics_changelog/cls.md

@@ -8,6 +8,8 @@ This is a list of changes to [Cumulative Layout Shift](https://web.dev/cls).
     * an element and its descendants if they move together, and
     * inline elements and texts in a block after a shifted text.
   These changes will affect layout instability score for the specific cases.
+* Chrome 85
+  * Metric definition improvement: [Cumulative Layout Shift ignores layout shifts from video slider thumb](2020_06_cls.md)
 * Chrome 79
   * Metric is elevated to stable; changes in metric definition will be reported in this log.
 * Chrome 77

docs/speed/metrics_changelog/fcp.md

@@ -2,6 +2,10 @@
 
 This is a list of changes to [First Contentful Paint](https://web.dev/fcp).
 
+* Chrome 86
+  * Metric definition improvements: [First Contentful Paint reported for videos, webgl2 canvases, and non-contentful SVG](2020_07_fcp.md)
+* Chrome 84
+  * Metric definition improvement: [First Contentful Paint not reported for opacity:0](2020_06_fcp.md)
 * Chrome 77
   * Metric definition improvement: [First Contentful Paint ending switches from swap time to presentation time](2019_12_fcp.md)
   * Chrome performance regression: [First Contentful Paint regression (recovered in Chrome 78)](2019_12_fcp.md)

docs/speed/metrics_changelog/lcp.md

@@ -2,6 +2,8 @@
 
 This is a list of changes to [Largest Contentful Paint](https://web.dev/lcp).
 
+* Chrome 86
+  * Metric definition improvement: [Largest Contentful Paint ignores paints with opacity 0](2020_08_lcp.md)
 * Chrome 83
   * Metric definition improvement: [Largest Contentful Paint measurement stops at first input or scroll](2020_05_lcp.md)
   * Metric definition improvement: [Largest Contentful Paint properly accounts for visual size of background images](2020_05_lcp.md)