Raise the priority of the release build task for WPT on macOS

… since other time-sensitive tasks depend on them.

Note: we need to be careful with task priorities,
especially in worker pools with limited capacity,
since they are absolute and can cause starvation:

https://docs.taskcluster.net/docs/manual/tasks/priority

etc/taskcluster/decision_task.py

@@ -608,10 +608,11 @@ def update_wpt():
     )
 
 
-def macos_release_build(args=""):
+def macos_release_build(args="", priority=None):
     return (
         macos_build_task("Release build")
         .with_treeherder("macOS x64", "Release")
+        .with_priority(priority)
         .with_script("\n".join([
             "./mach build --release --verbose " + args,
             "./etc/ci/lockfile_changed.sh",
@@ -627,7 +628,7 @@ def macos_release_build(args=""):
 
 
 def macos_wpt():
-    build_task = macos_release_build("--with-debug-assertions")
+    build_task = macos_release_build("--with-debug-assertions", priority="high")
     def macos_run_task(name):
         task = macos_task(name).with_python2()
         return with_homebrew(task, ["etc/taskcluster/macos/Brewfile-gstreamer"])

etc/taskcluster/decisionlib.py

@@ -144,6 +144,7 @@ class Task:
         self.routes = []
         self.extra = {}
         self.treeherder_required = False
+        self.priority = None  # Defaults to 'lowest'
 
     # All `with_*` methods return `self`, so multiple method calls can be chained.
     with_description = chaining(setattr, "description")
@@ -153,6 +154,7 @@ class Task:
     with_deadline_in = chaining(setattr, "deadline_in")
     with_expires_in = chaining(setattr, "expires_in")
     with_index_and_artifacts_expire_in = chaining(setattr, "index_and_artifacts_expire_in")
+    with_priority = chaining(setattr, "priority")
 
     with_dependencies = chaining(append_to_attr, "dependencies")
     with_scopes = chaining(append_to_attr, "scopes")
@@ -248,6 +250,7 @@ class Task:
             scopes=scopes,
             routes=routes,
             extra=self.extra,
+            priority=self.priority,
         )
 
         task_id = taskcluster.slugId()