Update WinRT Sensor flag expiration date

This CL updates the enable-winrt-sensor-implementation flag to expire
on M84 instead of M80 since the experimentation is still ongoing.

Bug: 958266
Change-Id: Icec793cc518958454a6040948ed2fd3d7a2a6c36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918093
Commit-Queue: Wenjie Shi <wensh@microsoft.com>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715561}

chrome/browser/flag-metadata.json

@@ -2154,7 +2154,7 @@
   {
     "name": "enable-winrt-sensor-implementation",
     "owners": [ "wensh@microsoft.com" ],
-    "expiry_milestone": 80
+    "expiry_milestone": 84
   },
  {
     "name": "enable-zero-copy",