Bug 1514087 - Rebuild custom d8 on a regular basis. r=jmaher

This patch adds a cron task to regularly schedule an update to the custom d8/v8 version in use in jsshell benchmarks.

Differential Revision: https://phabricator.services.mozilla.com/D15256

--HG--
extra : moz-landing-system : lando

.cron.yml

@@ -99,6 +99,17 @@ jobs:
               mozilla-central: [{hour: 10, minute: 0}]
               # No default
 
+    - name: customv8-update
+      job:
+          type: decision-task
+          treeherder-symbol: customv8
+          target-tasks-method: customv8_update
+      run-on-projects:
+          - mozilla-central
+      when:
+          by-project:
+              mozilla-central: [{hour: 10, minute: 30}]
+
     - name: bouncer-check
       job:
           type: decision-task

taskcluster/taskgraph/target_tasks.py

@@ -570,6 +570,12 @@ def target_tasks_searchfox(full_task_graph, parameters, graph_config):
             'searchfox-win64-searchfox/debug']
 
 
+@_target_task('customv8_update')
+def target_tasks_customv8_update(full_task_graph, parameters, graph_config):
+    """Select tasks required for building latest d8/v8 version."""
+    return ['toolchain-linux64-custom-v8']
+
+
 @_target_task('pipfile_update')
 def target_tasks_pipfile_update(full_task_graph, parameters, graph_config):
     """Select the set of tasks required to perform nightly in-tree pipfile updates