Bug 1372721 - Add a "trunk" alias for run-on-projects; r=dustin

{central, autoland, inbound} are logically treated as a single unit for
many tasks and policies. Let's formalize that collection via a "trunk"
alias.

MozReview-Commit-ID: H4JPTyu2J2F

taskcluster/taskgraph/util/attributes.py

@@ -10,6 +10,8 @@ INTEGRATION_PROJECTS = {
     'autoland',
 }
 
+TRUNK_PROJECTS = INTEGRATION_PROJECTS | {'mozilla-central',}
+
 RELEASE_PROJECTS = {
     'mozilla-central',
     'mozilla-aurora',
@@ -74,4 +76,8 @@ def match_run_on_projects(project, run_on_projects):
     if 'release' in run_on_projects:
         if project in RELEASE_PROJECTS:
             return True
+    if 'trunk' in run_on_projects:
+        if project in TRUNK_PROJECTS:
+            return True
+
     return project in run_on_projects