Bug 1554608 Schedule raptor fennec v64 tests in CI r=dustin,tomprince

Ran locally `./mach taskgraph cron --head-repository=https://hg.mozilla.org/mozilla-central --project=mozilla-central --level=1` and got no errors:
```
 0:00.54 using current time for params['time']; try setting $CRON_TIME to a timestamp
 0:00.54 calculated cron schedule time is 2019-05-27 09:30:00
 0:00.56 not running cron job bouncer-check
 0:00.56 not running cron job chromium-update
 0:00.56 not running cron job customv8-update
 0:00.56 not running cron job nightly-android
 0:00.56 not running cron job nightly-desktop
 0:00.56 not running cron job nightly-desktop-linux
 0:00.56 not running cron job nightly-desktop-osx
 0:00.56 not running cron job nightly-desktop-win32
 0:00.56 not running cron job nightly-desktop-win64
 0:00.56 not running cron job nightly-desktop-win64-aarch64
 0:00.56 not running cron job periodic-update
 0:00.56 not running cron job pipfile-update
 0:00.56 not running cron job searchfox-index
** 0:00.56 not running cron job tp6m-fennec-v64**
```
Feel free to add to this review whoever is relevant.

Differential Revision: https://phabricator.services.mozilla.com/D32674

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

.cron.yml

@@ -83,6 +83,17 @@ jobs:
               mozilla-central: [{hour: 10, minute: 0}]
               # No default
 
+    - name: tp6m-fennec-v64
+      job:
+          type: decision-task
+          treeherder-symbol: f64
+          target-tasks-method: fennec_v64
+          include-push-tasks: true
+      run-on-projects:
+          - mozilla-central
+      when:
+          - {weekday: 'Saturday', hour: 5, minute: 0}
+
     - name: searchfox-index
       job:
           type: decision-task

taskcluster/taskgraph/target_tasks.py

@@ -453,6 +453,25 @@ def target_tasks_nightly_geckoview(full_task_graph, parameters, graph_config):
     return [l for l, t in full_task_graph.tasks.iteritems() if filter(t)]
 
 
+@_target_task('fennec_v64')
+def target_tasks_fennec_v64(full_task_graph, parameters, graph_config):
+    """
+    Select tasks required for running tp6m fennec v64 tests
+    """
+    def filter(task):
+        platform = task.attributes.get('build_platform')
+        attributes = task.attributes
+
+        if platform and 'android' not in platform:
+            return False
+        if attributes.get('unittest_suite') != 'raptor':
+            return False
+        if '-fennec64-' in attributes.get('raptor_try_name'):
+            return True
+
+    return [l for l, t in full_task_graph.tasks.iteritems() if filter(t)]
+
+
 def make_desktop_nightly_filter(platforms):
     """Returns a filter that gets all nightly tasks on the given platform."""
     def filter(task, parameters):