Bug 1657769 - [ci] Run talos bcv on -qr platforms when software webrender is enabled, r=jmaher

From mwoodrow:

bcv is 'basic compositor video', which explicitly disables acceleration using a
pref and then runs some video tests. For normal -qr platforms this isn't
useful, since the pref would disable WebRender, and it'd be an identical
configuation to the non-qr variant.

When software webrender is enabled, disabling acceleration won't disable WR,
and test remains useful (testing video performance of the software webrender
configuration).

The baseline I want to compare against here is the non-qr bcv, to see if
webrender-software is worse than layers-software.

Differential Revision: https://phabricator.services.mozilla.com/D88192

taskcluster/ci/test/talos.yml

@@ -60,7 +60,10 @@ talos-bcv:
             default: ['fission']
     run-on-projects:
         by-test-platform:
-            .*(?:shippable-qr|-qr)/.*: []
+            .*(?:shippable-qr|-qr)/.*:
+                by-variant:
+                    webrender-sw: ['trunk']
+                    default: []
             (linux|windows|macos)(?!.*shippable)(?!.*-qr).*: []
             default: ['mozilla-beta', 'trunk']
     tier:

taskcluster/taskgraph/transforms/tests.py

@@ -387,6 +387,7 @@ test_description_schema = Schema({
     Optional('run-on-projects'): optionally_keyed_by(
         'test-platform',
         'test-name',
+        'variant',
         Any([text_type], 'built-projects')),
 
     # When set only run on projects where the build would already be running.
@@ -1054,7 +1055,7 @@ def handle_keyed_by(config, tasks):
     ]
     for task in tasks:
         for field in fields:
-            resolve_keyed_by(task, field, item_name=task['test-name'],
+            resolve_keyed_by(task, field, item_name=task['test-name'], defer=['variant'],
                              project=config.params['project'])
         yield task
 
@@ -1366,6 +1367,19 @@ def split_variants(config, tasks):
             yield merge(taskv, variant.get('merge', {}))
 
 
+@transforms.add
+def handle_keyed_by_variant(config, tasks):
+    """Resolve fields that can be keyed by platform, etc."""
+    fields = [
+        'run-on-projects',
+    ]
+    for task in tasks:
+        for field in fields:
+            resolve_keyed_by(task, field, item_name=task['test-name'],
+                             variant=task['attributes'].get('unittest_variant'))
+        yield task
+
+
 @transforms.add
 def handle_fission_attributes(config, tasks):
     """Handle run_on_projects for fission tasks."""