Extend WebGL 2.0 Compute flag expiry to M91.

Still being used for prototyping; replacement not yet available.

Tbr: jdarpinian@chromium.org
Bug: 859249
Change-Id: I7f435ed1827634c89cf5238615dcc90d56961a63
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417462Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808195}

chrome/browser/flag-metadata.json

@@ -2345,7 +2345,7 @@
   {
     "name": "enable-webgl2-compute-context",
     "owners": [ "webgl-team@google.com" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 91
   },
   {
     "name": "enable-webrtc-capture-multi-channel-audio-processing",