Disable thread-safety-reference-return warnings in libwebrtc

rdar://126802873
https://bugs.webkit.org/show_bug.cgi?id=274508

Reviewed by David Kilzer.

We disable thread-safety-reference-return as libwebrtc is not yet ready for this.
A few places would need additional annotations, and some other places are checking that the method is called in the right thread,
not that a mutex is held.

* Source/ThirdParty/libwebrtc/Configurations/Base.xcconfig:

Canonical link: https://commits.webkit.org/279140@main

Source/ThirdParty/libwebrtc/Configurations/Base.xcconfig

@@ -73,11 +73,11 @@ GCC_WARN_UNUSED_FUNCTION = YES;
 GCC_WARN_UNUSED_VARIABLE = YES;
 CLANG_WARN_SUSPICIOUS_MOVE = YES;
 PREBINDING = NO;
-WARNING_CFLAGS = $(inherited) $(WK_FIXME_WARNING_CFLAGS) -Wexit-time-destructors -Wglobal-constructors -Wthread-safety;
+WARNING_CFLAGS = $(inherited) -Wthread-safety $(WK_FIXME_WARNING_CFLAGS) -Wexit-time-destructors -Wglobal-constructors;
 
 // Remove WK_FIXME_WARNING_CFLAGS once all warnings are fixed.
 // -Wno-unknown-warning-option added for -Wno-unused-but-set-parameter.
-WK_FIXME_WARNING_CFLAGS = -Wno-conditional-uninitialized -Wno-missing-field-initializers -Wno-sign-compare -Wno-undef -Wno-unknown-warning-option -Wno-unused-but-set-parameter -Wno-unused-parameter -Wno-array-parameter -Wno-unused-but-set-variable;
+WK_FIXME_WARNING_CFLAGS = -Wno-conditional-uninitialized -Wno-missing-field-initializers -Wno-sign-compare -Wno-undef -Wno-unknown-warning-option -Wno-unused-but-set-parameter -Wno-unused-parameter -Wno-array-parameter -Wno-unused-but-set-variable -Wno-thread-safety-reference-return;
 
 ENTITLEMENTS_REQUIRED = $(ENTITLEMENTS_REQUIRED_USE_INTERNAL_SDK_$(USE_INTERNAL_SDK))
 ENTITLEMENTS_REQUIRED_USE_INTERNAL_SDK_ = NO;