Enable Background purge on mac in fieldtrial_testing_config.json.

This is currently being experimented on, with very positive results (see
https://uma.googleplex.com/p/chrome/variations?sid=0fbe3f0abe9a6530dbf34357a30ed0f3
for the dev/canary data).

Enable it, in preparation for the beta experiment.

Bug: 1109208
Change-Id: Iffade4ec360dbda6fcdb065d9e640517f38aa98f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340046Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795484}

testing/variations/fieldtrial_testing_config.json

@@ -1044,6 +1044,21 @@
             ]
         }
     ],
+    "BackgroundPurgeOnMac": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "BackgrounPurgeOnMac",
+                    "enable_features": [
+                        "PurgeRendererMemoryWhenBackgrounded"
+                    ]
+                }
+            ]
+        }
+    ],
     "BackgroundSync": [
         {
             "platforms": [