[UI-side compositing] [GPU Process] Emailing a webpage from Safari causes the tab to stop rendering

[whsieh]

e74572e

[UI-side compositing] [GPU Process] Emailing a webpage from Safari causes the tab to stop rendering
https://bugs.webkit.org/show_bug.cgi?id=254683
rdar://107329972

Reviewed by Simon Fraser.

When sharing a webpage via Mail, Safari first generates a web archive from the existing web view's
content, and then creates an offscreen web view *backed by the tab's existing web process*, which we
then use to load the web archive and perform reader extraction heuristics.

However, when the DOM GPU process experimental feature flag is enabled for Safari tabs, it only
affects the web view used for rendering the main page content; importantly, this related web view
does not have DOM GPUP enabled.

When subsequently loading the web archive data via this reader web view, we end up *disabling* GPUP
for DOM rendering in the original tab's web process, due to the following code:

```
bool usingGPUProcessForDOMRendering = m_shouldRenderDOMInGPUProcess && DrawingArea::supportsGPUProcessRendering(m_drawingAreaType);
WebProcess::singleton().setUseGPUProcessForDOMRendering(usingGPUProcessForDOMRendering);
```

The current implementation of GPUP for DOM rendering doesn't handle on-the-fly GPUP disablement —
and in fact, in the case where IOKit has already been blocked in the web process, it's impossible to
disable GPUP at all once a page has been created with all the relevant GPUP flags enabled.

As such, we mitigate this by simply forcing DOM GPUP to be enabled (regardless of preferences state)
for a web view, if its related web view already has DOM GPUP enabled. Once DOM GPUP is enabled by
default, this won't be an issue anyway, since all web views will be created with consistent feature
flag statuses.

Test: GPUProcess.GPUProcessForDOMRenderingCarriesOverFromRelatedPage

* Source/WebKit/UIProcess/Cocoa/WebPageProxyCocoa.mm:
(WebKit::WebPageProxy::useGPUProcessForDOMRenderingEnabled const):
* Tools/TestWebKitAPI/Tests/WebKitCocoa/GPUProcess.mm:
(-[NSUserDefaults swizzled_objectForKey:]):
(TestWebKitAPI::EnableUISideCompositingScope::EnableUISideCompositingScope):
(TestWebKitAPI::EnableUISideCompositingScope::~EnableUISideCompositingScope):

Add a new RAII helper class to temporarily swizzle out `-objectForKey:`, in order to forcibly enable
UI-side compositing for this test, as it's necessary to exercise the bug fix. Note that I'm using
swizzling here to avoid also enabling UI-side compositing for downstream API tests, even if the test
crashes or times out.

(TestWebKitAPI::TEST):
(convertToCGImage): Deleted.
(getPixelIndex): Deleted.
(TEST): Deleted.
(runMemoryPressureExitTest): Deleted.
(waitUntilCaptureState): Deleted.

Canonical link: https://commits.webkit.org/262296@main

404f156

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	🧪 ios-wk2	✅ 🧪 api-mac	✅ 🛠 gtk
	✅ 🧪 api-ios	✅ 🧪 mac-wk1	🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-wk2	🧪 api-gtk
	✅ 🛠 tv-sim	🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
✅ 🛠 🧪 merge	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 404f156)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	✅ 🧪 wpe-wk2
✅ 🧪 webkitperl	🧪 ios-wk2	✅ 🧪 api-mac	✅ 🛠 gtk
	✅ 🧪 api-ios	✅ 🧪 mac-wk1	🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-wk2	🧪 api-gtk
	✅ 🛠 tv-sim	🧪 mac-AS-debug-wk2
	✅ 🛠 watch	✅ 🧪 mac-wk2-stress
✅ 🛠 🧪 merge	✅ 🛠 watch-sim

[whsieh]

Thanks for the review!

[webkit-commit-queue]

Committed 262296@main (e74572e): https://commits.webkit.org/262296@main

Reviewed commits have been landed. Closing PR #12130 and removing active labels.