[WebKitSettings] Add API for OpportunisticSweepingAndGarbageCollectio…

[asurdej-comcast]

…nEnabled

We observe increased WebKit 2.46 average memory usage (vs 2.38) with OpportunisticSweepingAndGarbageCollectionEnabled. In case of LightningApps (webgl/non composited webgl) this option effectively disables GC timers in some cases (FullGCActivityCallback and EdenGCActivityCallback) and prevents them from running.
Initially reported as memory during video playback but I can observe it also with UI scrolling

The only source of GC is Heap.cpp:

2026-02-05 06:50:18 Requesting GC because bytes allocated this cycle: 855941 exceed bytes allowed: 817924 (critical) normal bytes: 715034 oversized bytes: 140907 last oversized: 66063
2026-02-05 06:50:18 [GC<0x8c014088>: START M 1020kb => FullCollection, v=0kb (C:0 M:0 P1:0 P2:0 P3:0) o=0 b=87 i#1:N<CsMsrShDMsm(0)> 21+0 v=140kb (C:28 M:0 P1:17 P2:27 P3:66) o=2 b=87 i#2:N<WsOJwJIT Worklists:
2026-02-05 06:50:18 Collector: [], Mutator: []> 0+0 v=140kb (C:28 M:0 P1:17 P2:27 P3:66) o=2 b=87 i#3:P<WsOJwJIT Worklists:
2026-02-05 06:50:18 Collector: [], Mutator: []MsrShCsMsm(0)DCbDomo> => 234kb, p=5.890000ms (max 173.480000), cycle 5.852000ms END]
2026-02-05 06:50:18 GC END!
2026-02-05 06:50:18 [GC<0x8c014088>: finalize 0.225000ms]

GC timers are blocked by OpportunisticTaskScheduler, where isBusyForTimerBasedGC() is always true because of requestAnimationFrame() callback that is set over and over for LightningApp and it considered as imminently scheduled task.

ScriptedAnimationController::CallbackId ScriptedAnimationController::registerCallback(Ref<RequestAnimationFrameCallback>&& callback)
{

    RefPtr<ImminentlyScheduledWorkScope> workScope;
    if (RefPtr page = this->page()) {
        workScope = page->opportunisticTaskScheduler().makeScheduledWorkScope();

Here is how WebProcess memory usage looks for hbomax app steady video playback with webkit 2.38:
marked moments are FullGCActivityCallback, beside that EdenGCActivityCallback triggers GC every 10sec

And here is how it looks with 2.46 where GC is invoked at spikes only, every ~1min. Full gc is marked with a red circle while other points are Eden GC

Disabling OpportunisticSweepingAndGarbageCollectionEnabled brings back wpe-2.38 behavior.

WebKit/WebKit@5c9eac4
WebKit/WebKit#20688

[magomez]

Amazing findings @asurdej-comcast !! Thanks for the debug!!