Promote webgl_conformance_tests out of experiment on CrOS VM CQ bot.

Little to no flakes for the test on the CQ since adding it as experiment:
https://findit-for-me.appspot.com/p/chromium/flake-portal/flakes?flake_filter=suite%3A%3Awebgl_conformance_tests%40builder%3A%3Achromeos-amd64-generic-rel

Bug: 957807
Change-Id: I5c9a42dfeaad3a769f9f7827b07c612198e4cf2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636461Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664777}

testing/buildbot/chromium.chromiumos.json

@@ -535,7 +535,6 @@
           "--remote=127.0.0.1",
           "--remote-ssh-port=9222"
         ],
-        "experiment_percentage": 100,
         "isolate_name": "telemetry_gpu_integration_test",
         "merge": {
           "args": [],

testing/buildbot/test_suite_exceptions.pyl

@@ -1560,8 +1560,6 @@
         ]
       },
       'chromeos-amd64-generic-rel': {
-        # TODO(crbug.com/957807): Remove from experimental.
-        'experiment_percentage': 100,
         'swarming': {
           'shards': 6,
         },