Change the expiry of fling animation flag

Changes for new fling animation curve are still
ongoing. Thus extending the expiry milestone of
"enable-experimental-fling-animation" flag to M81.

Change-Id: Ia2ef834ff1bc88a46b2ff979a94ff981f55d7306
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918144Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sarthak Shah <sarsha@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#715908}

chrome/browser/flag-metadata.json

@@ -1322,7 +1322,7 @@
   {
     "name": "enable-experimental-fling-animation",
     "owners": [ "sarsha@microsoft.com" ],
-    "expiry_milestone": 80
+    "expiry_milestone": 81
   },
   {
     "name": "enable-experimental-kernel-vm-support",