Don't directly expose s_instanceHandle on Windows

https://bugs.webkit.org/show_bug.cgi?id=248503

Reviewed by Alex Christensen and Fujii Hironori.

All accesses of `s_instanceHandle` used the getter/setter functions so
just remove it from the header. Export the functions so WebCore can be
turned into a shared library.

* Source/WebCore/platform/win/WebCoreInstanceHandle.cpp:
* Source/WebCore/platform/win/WebCoreInstanceHandle.h:

Canonical link: https://commits.webkit.org/257183@main

Source/WebCore/platform/win/WebCoreInstanceHandle.cpp

@@ -28,6 +28,17 @@
 
 namespace WebCore {
 
-HINSTANCE s_instanceHandle;
+// The global DLL or application instance used for all windows.
+HINSTANCE s_instanceHandle = nullptr;
+
+void setInstanceHandle(HINSTANCE instanceHandle)
+{
+    s_instanceHandle = instanceHandle;
+}
+
+HINSTANCE instanceHandle()
+{
+    return s_instanceHandle;
+}
 
 } // namespace WebCore

Source/WebCore/platform/win/WebCoreInstanceHandle.h

@@ -23,19 +23,13 @@
  * THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef WebCoreInstanceHandle_h
-#define WebCoreInstanceHandle_h
+#pragma once
 
 typedef struct HINSTANCE__* HINSTANCE;
 
 namespace WebCore {
 
-// The global DLL or application instance used for all windows.
-extern HINSTANCE s_instanceHandle;
-
-inline void setInstanceHandle(HINSTANCE instanceHandle) { s_instanceHandle = instanceHandle; }
-inline HINSTANCE instanceHandle() { return s_instanceHandle; }
+WEBCORE_EXPORT void setInstanceHandle(HINSTANCE);
+WEBCORE_EXPORT HINSTANCE instanceHandle();
 
 }
-
-#endif // WebCoreInstanceHandle_h