Extend expiery milestore for arc-file-picker-experiment

There are a few lower priority differences with the CrOS file
picker so the flexibility to switch is currently still useful.

BUG=None
TEST=trybot

Change-Id: Ib05ee202726ebe460a2684fbbdbe26b79bdb3ab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014200Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
Auto-Submit: Satoshi Niwa <niwa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733879}

chrome/browser/flag-metadata.json

@@ -159,7 +159,7 @@
   {
     "name": "arc-file-picker-experiment",
     "owners": [ "niwa" ],
-    "expiry_milestone": 80
+    "expiry_milestone": 88
   },
   {
     "name": "arc-graphics-buffer-visualization-tool",