Build fix for MediaRecorderPrivateEncoder in recent clang toolchains

[geoffreygaren]

8f78933

Build fix for MediaRecorderPrivateEncoder in recent clang toolchains
https://bugs.webkit.org/show_bug.cgi?id=282549
rdar://139216339

Reviewed by Chris Dumez.

MediaRecorderPrivateEncoder tells the compiler "You can only use m_mimeType
inside a scope in which you assertIsMainThread()".

Recent versions of clang notice that this scope ends when a function returns,
so, in the mimeType() accessor, it is an error to return a reference to m_mimeType.

The simplest fix for now seems to be to return String instead of String&.

Latern on, maybe we can annotate the accessor in a way that shows the compiler
that assertIsMainThread() continues to be true after we return.

* Source/WebCore/platform/mediarecorder/MediaRecorderPrivateEncoder.h:

Canonical link: https://commits.webkit.org/286111@main

9c644f1

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2	❌ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	🧪 mac-wk1	🛠 wpe-cairo
	🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	🧪 mac-AS-debug-wk2	🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2
✅ 🛠 🧪 unsafe-merge	✅ 🛠 tv	🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on previous version of this PR (hash 37da508)

Details

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	❌ 🛠 ios	❌ 🛠 mac	🛠 wpe	🛠 win
✅ 🧪 bindings	🛠 ios-sim	❌ 🛠 mac-AS-debug	🧪 wpe-wk2	🧪 win-tests
✅ 🧪 webkitperl	🧪 ios-wk2	❌ 🧪 api-mac	🧪 api-wpe
	🧪 ios-wk2-wpt	❌ 🧪 mac-wk1	🛠 wpe-cairo
	🧪 api-ios	❌ 🧪 mac-wk2	✅ 🛠 gtk
	🛠 vision	❌ 🧪 mac-AS-debug-wk2	⏳ 🧪 gtk-wk2
	🛠 vision-sim	❌ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	⏳ 🧪 vision-wk2	❌ 🧪 mac-intel-wk2
	🛠 tv	🛠 mac-safer-cpp
	🛠 tv-sim
	🛠 watch
	🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 9c644f1)

Details

Misc	iOS, visionOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 win
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2	❌ 🧪 win-tests
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	🧪 mac-wk1	🛠 wpe-cairo
	🧪 api-ios	✅ 🧪 mac-wk2	✅ 🛠 gtk
	✅ 🛠 vision	🧪 mac-AS-debug-wk2	🧪 gtk-wk2
	✅ 🛠 vision-sim	✅ 🧪 mac-wk2-stress	✅ 🧪 api-gtk
	✅ 🧪 vision-wk2	✅ 🧪 mac-intel-wk2
✅ 🛠 🧪 unsafe-merge	✅ 🛠 tv	🛠 mac-safer-cpp
	✅ 🛠 tv-sim
	🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 286111@main (8f78933): https://commits.webkit.org/286111@main

Reviewed commits have been landed. Closing PR #36135 and removing active labels.