Group WPT chunks together in Treeherder

etc/taskcluster/decision_task.py

@@ -708,7 +708,12 @@ def wpt_chunks(platform, make_chunk_task, build_task, total_chunks, processes,
                 total_chunks,
                 width=len(str(total_chunks)),
             ))
-            .with_treeherder(platform, "%sWPT-%s" % (job_id_prefix, this_chunk))
+            .with_treeherder(
+                platform,
+                "WPT-%s" % this_chunk,
+                group_symbol=job_id_prefix + "WPT",
+                group_name=name_prefix + "web-platform-tests"
+            )
             .with_curl_artifact_script(build_task, "target.tar.gz")
             .with_script("tar -xzf target.tar.gz")
             .with_index_and_artifacts_expire_in(log_artifacts_expire_in)

etc/taskcluster/decisionlib.py

@@ -172,7 +172,7 @@ class Task:
         self.treeherder_required = True
         return self
 
-    def with_treeherder(self, category, symbol):
+    def with_treeherder(self, category, symbol, group_name=None, group_symbol=None):
         assert len(symbol) <= 25, symbol
         self.name = "%s: %s" % (category, self.name)
 
@@ -185,12 +185,16 @@ class Task:
         platform = parts[0]
         labels = parts[1:] or ["_"]
 
-        # https://docs.taskcluster.net/docs/reference/integrations/taskcluster-treeherder/docs/task-treeherder-config
-        self.with_extra(treeherder={
+        # https://github.com/mozilla/treeherder/blob/master/schemas/task-treeherder-config.yml
+        self.with_extra(treeherder=dict_update_if_truthy(
+            {
                 "machine": {"platform": platform},
                 "labels": labels,
                 "symbol": symbol,
-        })
+            },
+            groupName=group_name,
+            groupSymbol=group_symbol,
+        ))
 
         if CONFIG.treeherder_repository_name:
             assert CONFIG.git_sha