Update WebGL roller to use linux-angle-rel.

linux_angle_rel_ng is on the way out.

Bug: 822310
Change-Id: I1989e860462bd13f5648c13f933225f3186d347e
Tbr: kbr@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566111Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650351}

tools/roll_webgl_conformance.py

@@ -33,7 +33,7 @@ extra_trybots = [
   # in some non-default configurations.
   {
     "mastername": "luci.chromium.try",
-    "buildernames": ["linux_angle_rel_ng"]
+    "buildernames": ["linux-angle-rel"]
   },
   {
     "mastername": "luci.chromium.try",