Extend WebGL 2.0 Compute flag expiry to M85.

Tbr: jdarpinian@chromium.org
Bug: 859249
Change-Id: I973d272c1d99ba2c94e739eaba6759511afbfd78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015980Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734254}

chrome/browser/flag-metadata.json

@@ -2114,7 +2114,7 @@
   {
     "name": "enable-webgl2-compute-context",
     "owners": [ "webgl-team@google.com" ],
-    "expiry_milestone": 81
+    "expiry_milestone": 85
   },
   {
     "name": "enable-webnfc",