Compile errors around pasteboard when importing more UIKit.

https://bugs.webkit.org/show_bug.cgi?id=271424
rdar://125198774

Reviewed by Tim Horton and Aditya Keerthi.

Explicitly state that m_pasteboard is an AbstractPasteboard to deal with
compile errors arrising from importing more of UIKit.

* Source/WebCore/platform/PlatformPasteboard.h:

Canonical link: https://commits.webkit.org/276581@main

Source/WebCore/platform/PlatformPasteboard.h

@@ -38,6 +38,7 @@ OBJC_CLASS NSPasteboardItem;
 
 #if PLATFORM(IOS_FAMILY)
 OBJC_CLASS UIPasteboard;
+OBJC_PROTOCOL(AbstractPasteboard);
 #endif
 
 #if USE(LIBWPE)
@@ -123,7 +124,7 @@ class PlatformPasteboard {
     RetainPtr<NSPasteboard> m_pasteboard;
 #endif
 #if PLATFORM(IOS_FAMILY)
-    RetainPtr<id> m_pasteboard;
+    RetainPtr<AbstractPasteboard> m_pasteboard;
 #endif
 #if USE(LIBWPE)
     struct wpe_pasteboard* m_pasteboard;

Source/WebCore/platform/ios/PlatformPasteboardIOS.mm

@@ -54,6 +54,9 @@
 #define PASTEBOARD_SUPPORTS_ITEM_PROVIDERS (PLATFORM(IOS_FAMILY) && !(PLATFORM(WATCHOS) || PLATFORM(APPLETV)))
 #define PASTEBOARD_SUPPORTS_PRESENTATION_STYLE_AND_TEAM_DATA (PASTEBOARD_SUPPORTS_ITEM_PROVIDERS && !PLATFORM(MACCATALYST))
 
+@interface UIPasteboard () <AbstractPasteboard>
+@end
+
 namespace WebCore {
 
 PlatformPasteboard::PlatformPasteboard()