blink: Enable background renderer purge on macOS.

Stable 1% experiment shows very positive results on memory with no
adverse responsiveness/latency issues, so let's enable the feature by
default.

More specifically, 95th percentile footprints (from the experiment):
- Browser: -12%
- GPU process: -12%
- Per-renderer: -6%
- Total: -7%

And for discardable memory, -43%@95th percentile.

Also remove the testing config, since it is no longer needed.

Bug: 1109208, 926186
Change-Id: I99b1f8d36e2b754879ada98468a4464358652cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2403465Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805749}

testing/variations/fieldtrial_testing_config.json

@@ -661,21 +661,6 @@
             ]
         }
     ],
-    "BackgroundPurgeOnMac": [
-        {
-            "platforms": [
-                "mac"
-            ],
-            "experiments": [
-                {
-                    "name": "BackgrounPurgeOnMac",
-                    "enable_features": [
-                        "PurgeRendererMemoryWhenBackgrounded"
-                    ]
-                }
-            ]
-        }
-    ],
     "BackgroundSync": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -143,12 +143,11 @@ const base::Feature kPreviewsResourceLoadingHintsSpecificResourceTypes{
 // Perform a memory purge after a renderer is backgrounded. Formerly labelled as
 // the "PurgeAndSuspend" experiment.
 //
-// TODO(adityakeerthi): Disabled by default on Mac and Android for historical
+// TODO(https://crbug.com/926186): Disabled by default on Android for historical
 // reasons. Consider enabling by default if experiment results are positive.
-// https://crbug.com/926186
 const base::Feature kPurgeRendererMemoryWhenBackgrounded {
   "PurgeRendererMemoryWhenBackgrounded",
-#if defined(OS_MAC) || defined(OS_ANDROID)
+#if defined(OS_ANDROID)
       base::FEATURE_DISABLED_BY_DEFAULT
 #else
       base::FEATURE_ENABLED_BY_DEFAULT