Extend flag expiration for ongoing Sensor project.

The features are still under development. The flag is used for testing.

Change-Id: I974916c349775f0cb95938423d15e728c3121591
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421729
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809661}

chrome/browser/flag-metadata.json

@@ -1569,7 +1569,7 @@
   {
     "name": "enable-generic-sensor-extra-classes",
     "owners": [ "reillyg@chromium.org", "raphael.kubo.da.costa@intel.com" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 90
   },
   {
     "name": "enable-google-srp-isolated-prerender-nsp",