Extend expiry-milestone (compositor-threaded-scrollbar-scrolling).

This CL updates the exipry-milestone of the flag to 90. This feature is
a part of an ongoing effort for "scrolling unification". Even after the
feature "Compositor threaded scrollbar scrolling" is turned on by
default, being able to access this flag will be handy for quickly
debugging gesture based scrolling bugs.

Change-Id: Ie426aa07b2c2c0f7f501f0e843a797df6b871076
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1922649Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#716454}

chrome/browser/flag-metadata.json

@@ -507,7 +507,7 @@
   {
     "name": "compositor-threaded-scrollbar-scrolling",
     "owners": [ "arakeri@microsoft.com" ],
-    "expiry_milestone": 80
+    "expiry_milestone": 90
   },
   {
     "name": "context-menu-search-with-google-lens",