Update OOP-D flag expiry

The feature flag is still needed on Chrome OS until after M80 branch.

Bug: none
Change-Id: Ifc02c89cbdff9818f9fa48590a9c7e6e5da60038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919420Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715768}

chrome/browser/flag-metadata.json

@@ -2009,7 +2009,7 @@
   {
     "name": "enable-viz-display-compositor",
     "owners": [ "kylechar", "rjkroege" ],
-    "expiry_milestone": 79
+    "expiry_milestone": 81
   },
   {
     "name": "enable-viz-hit-test-surface-layer",