Extend smooth scrolling flag lifetime

This flag is useful both for debugging and accessibility. While we wait
for an accessibility UI we shouldn't remove the flag.

Bug: 817814
Change-Id: I85c8dbf2479f3ea36552d6b00794695f81284858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898331Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713522}

chrome/browser/flag-metadata.json

@@ -3262,7 +3262,7 @@
   {
     "name": "smooth-scrolling",
     "owners": [ "bokan", "input-dev" ],
-    "expiry_milestone": 80
+    "expiry_milestone": 85
   },
   {
     "name": "split-settings",