[iOS] ASSERTION FAILED: isMainThread() under WebKit::RemoteGraphicsCo…

…ntextGLProxy::RemoteGraphicsContextGLProxy

https://bugs.webkit.org/show_bug.cgi?id=255307
rdar://107905440

Reviewed by Matt Woodrow.

Switch GraphicsContextGLIdentifier to AtomicObjectIdentifier since new
identifiers get generated off the main thread.

* Source/WebKit/WebProcess/GPU/graphics/GraphicsContextGLIdentifier.h:

Canonical link: https://commits.webkit.org/262846@main

Source/WebKit/WebProcess/GPU/graphics/GraphicsContextGLIdentifier.h

@@ -32,7 +32,7 @@
 namespace WebKit {
 
 enum GraphicsContextGLIdentifierType { };
-using GraphicsContextGLIdentifier = ObjectIdentifier<GraphicsContextGLIdentifierType>;
+using GraphicsContextGLIdentifier = AtomicObjectIdentifier<GraphicsContextGLIdentifierType>;
 
 } // namespace WebKit