[On-device adaptive brightness] Extend flag expiry.

Bug: 881215
Test: unit_tests --gtest_filter=AboutFlagsTest*, AboutFlagsHistogramTest*
Change-Id: I268e9495dfd6f6f0fc9817e156ca4cd2624677b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918780Reviewed-by: Tony Yeoman <tby@chromium.org>
Commit-Queue: Jia Meng <jiameng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715590}

chrome/browser/flag-metadata.json

@@ -250,7 +250,7 @@
   {
     "name": "auto-screen-brightness",
     "owners": [ "jiameng", "napper" ],
-    "expiry_milestone": 79
+    "expiry_milestone": 90
   },
   {
     "name": "autofill-always-return-cloud-tokenized-card",