Extend enable-webgl2-compute-context expiry to M81.

Still being used by developers to try compute shaders on the web.

Bug: 859249
Change-Id: I92ddd9ca91722e50794d717d8551f3eef6e5805c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857279
Auto-Submit: Kenneth Russell <kbr@chromium.org>
Reviewed-by: James Darpinian <jdarpinian@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705337}

chrome/browser/flag-metadata.json

@@ -1973,7 +1973,7 @@
   {
     "name": "enable-webgl2-compute-context",
     "owners": [ "webgl-team@google.com" ],
-    "expiry_milestone": 77
+    "expiry_milestone": 81
   },
   {
     "name": "enable-webnfc",