mac: enables BlinkCompositorUseDisplayThreadPriority in testing config

I haven't actually enabled the experiment yet, will do that shortly.
I'm doing this to force perf bots to run with the feature.

BUG=1288931

Change-Id: Ib534673dac0d28a321060801052f1d7e55268a09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3401941
Auto-Submit: Scott Violet <sky@chromium.org>
Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/main@{#961194}

testing/variations/fieldtrial_testing_config.json

@@ -1637,6 +1637,21 @@
             ]
         }
     ],
+    "BlinkCompositorUseDisplayThreadPriority": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "BlinkCompositorUseDisplayThreadPriority"
+                    ]
+                }
+            ]
+        }
+    ],
     "BrowserJankinessProfiling": [
         {
             "platforms": [