Updating system-keyboard-lock flag expiration

This flag is still useful (and relied on for several tests) so we
don't want to remove it yet.  We are working on a second version
of the KeyboardLock spec which will allow callers to specify whether
they want to capture system keyboard events.  Once that spec is
signed off, we plan to implement the feature and remove this flag.

Change-Id: Ifd5d07c7643213eea3060deefe59659d3d0bcce8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429144
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810346}

chrome/browser/flag-metadata.json

@@ -4263,7 +4263,7 @@
   {
     "name": "system-keyboard-lock",
     "owners": [ "joedow", "garykac", "jamiewalch" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 95
   },
   {
     "name": "system-tray-mic-gain",