Extend the milestone expiry for the md rounded corners flag.

Bug: None
Change-Id: I80a296b678ce5ad7577c937035cd45a9bf54b30c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2488383Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819251}

chrome/browser/flag-metadata.json

@@ -1758,7 +1758,7 @@
   {
     "name": "enable-md-rounded-corners-on-dialogs",
     "owners": [ "tluk" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 90
   },
   {
     "name": "enable-media-feeds",