CrOS: Allow trim on memory pressure to M90

Bug=CHROMIUM:973963

Change-Id: Ib2fe85abd4fbe5dee7c6da48f7f1f5c25dd17a70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359740Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798821}

chrome/browser/flag-metadata.json

@@ -4309,7 +4309,7 @@
   {
     "name": "trim-on-memory-pressure",
     "owners": [ "bgeffon", "sonnyrao" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 90
   },
   {
     "name": "trust-tokens",