REGRESSION (266680@main): iOS PLUM ~0.75% increased memory use

https://bugs.webkit.org/show_bug.cgi?id=262575
rdar://114845469

Reviewed by Per Arne Vollan and Ryosuke Niwa.

Add a runtime-enabled setting to control whether or not the opportunistic task scheduler should be
enabled for the purposes of opportunistic sweeping and GC. This currently causes a memory regression
on PLUM, specifically on certain iPhone models.

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:
* Source/WebCore/page/OpportunisticTaskScheduler.cpp:
(WebCore::OpportunisticTaskScheduler::runLoopObserverFired):

Canonical link: https://commits.webkit.org/268882@main

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -4914,6 +4914,22 @@ OffscreenCanvasInWorkersEnabled:
       "PLATFORM(COCOA)": true
       default: false
 
+# FIXME: Reenable this on iOS once we can mitigate impact on memory use.
+OpportunisticSweepingAndGarbageCollectionEnabled:
+  type: bool
+  status: internal
+  humanReadableName: "Opportunistic Sweeping and GC"
+  humanReadableDescription: "Enable Opportunistic Sweeping and GC"
+  category: javascript
+  defaultValue:
+    WebKitLegacy:
+      default: false
+    WebKit:
+      "PLATFORM(IOS_FAMILY)": false
+      default: true
+    WebCore:
+      default: false
+
 OpusDecoderEnabled:
   type: bool
   status: mature

Source/WebCore/page/OpportunisticTaskScheduler.cpp

@@ -101,6 +101,9 @@ void OpportunisticTaskScheduler::runLoopObserverFired()
     if (UNLIKELY(!page))
         return;
 
+    if (!page->settings().opportunisticSweepingAndGarbageCollectionEnabled())
+        return;
+
     page->performOpportunisticallyScheduledTasks(deadline);
 }