Extend arc-file-picker-experiment

This flag will be still useful as an escape hatch or as a debugging
option after the launch of ARC R.

BUG=None
TEST=trybot

Change-Id: I060be7354d8636c6bb78cc6a20601f7cbdde2120
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2596572
Auto-Submit: Satoshi Niwa <niwa@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839301}

chrome/browser/flag-metadata.json

@@ -157,7 +157,7 @@
   {
     "name": "arc-file-picker-experiment",
     "owners": [ "niwa" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 95
   },
   {
     "name": "arc-native-bridge-64bit-support-experiment",