Web MIDI: Extends use-winrt-midi-api flag until m83

I haven't worked on enabling this for a while. But let me
reschedule it this work sometime at the 2020Q1.

Bug: 912851
Change-Id: I9fb04d5ee261ae8c83e5ae3ff4367a8bc99e7805
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1862594Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Takashi Toyoshima <toyoshim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705907}

chrome/browser/flag-metadata.json

@@ -3409,7 +3409,7 @@
   {
     "name": "use-winrt-midi-api",
     "owners": [ "toyoshim" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 83
   },
   {
     "name": "user-activation-v2",