Fix unused function warning in RemoteRenderingBackend.cpp

[donny-dont]

a6482d7

Fix unused function warning in RemoteRenderingBackend.cpp
https://bugs.webkit.org/show_bug.cgi?id=255410

Unreviewed build fix.

The function `handleFromBuffer` was only used in `PLATFORM(COCOA)` code.

* Source/WebKit/GPUProcess/graphics/RemoteRenderingBackend.cpp:

Canonical link: https://commits.webkit.org/262930@main

7836501

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

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 39d312f)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	🛠 ios	✅ 🛠 mac	⏳ 🛠 wpe	⏳ 🛠 wincairo
	⏳ 🛠 ios-sim	⏳ 🛠 mac-AS-debug	⏳ 🧪 wpe-wk2
✅ 🧪 webkitperl	⏳ 🧪 ios-wk2	🧪 api-mac	✅ 🛠 gtk
	⏳ 🧪 ios-wk2-wpt	🧪 mac-wk1	⏳ 🧪 gtk-wk2
⏳ 🛠 🧪 jsc	⏳ 🧪 api-ios	🧪 mac-wk2	⏳ 🧪 api-gtk
⏳ 🛠 🧪 jsc-arm64	⏳ 🛠 tv	⏳ 🧪 mac-AS-debug-wk2
	⏳ 🛠 tv-sim	🧪 mac-wk2-stress
	⏳ 🛠 watch
	⏳ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 7836501)

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

[webkit-commit-queue]

Committed 262930@main (a6482d7): https://commits.webkit.org/262930@main

Reviewed commits have been landed. Closing PR #12707 and removing active labels.