msxml3/xmldoc: Fix IPersistStreamInit::GetClassID() for IXMLDocument.

dlls/msxml3/tests/xmldoc.c

@@ -361,6 +361,7 @@ static void test_persiststreaminit(void)
     HRESULT hr;
     ULARGE_INTEGER size;
     CHAR path[MAX_PATH];
+    CLSID id;
 
     hr = CoCreateInstance(&CLSID_XMLDocument, NULL, CLSCTX_INPROC_SERVER,
                           &IID_IXMLDocument, (LPVOID*)&doc);
@@ -380,6 +381,9 @@ static void test_persiststreaminit(void)
     hr = IPersistStreamInit_Save(psi, NULL, FALSE);
     todo_wine ok(hr == E_INVALIDARG, "Expected E_INVALIDARG, got %08x\n", hr);
 
+    hr = IPersistStreamInit_GetClassID(psi, NULL);
+    ok(hr == E_POINTER, "Expected E_INVALIDARG, got %08x\n", hr);
+
     create_xml_file("bank.xml");
     GetFullPathNameA("bank.xml", MAX_PATH, path, NULL);
     create_stream_on_file(&stream, path);
@@ -426,6 +430,11 @@ static void test_persiststreaminit(void)
     todo_wine ok(stat.cbSize.QuadPart > 0, "Expected >0\n");
     IStream_Release(stream);
 
+    memset(&id, 0, sizeof(id));
+    hr = IPersistStreamInit_GetClassID(psi, &id);
+    ok(hr == S_OK, "Expected S_OK, got %08x\n", hr);
+    ok(IsEqualCLSID(&id, &CLSID_XMLDocument), "Expected CLSID_XMLDocument\n");
+
 cleanup:
     IPersistStreamInit_Release(psi);
     IXMLDocument_Release(doc);

dlls/msxml3/xmldoc.c

@@ -574,8 +574,13 @@ static ULONG WINAPI xmldoc_IPersistStreamInit_Release(
 static HRESULT WINAPI xmldoc_IPersistStreamInit_GetClassID(
     IPersistStreamInit *iface, CLSID *classid)
 {
-    FIXME("(%p,%p): stub!\n", iface, classid);
-    return E_NOTIMPL;
+    xmldoc *this = impl_from_IPersistStreamInit(iface);
+    TRACE("(%p,%p)\n", this, classid);
+
+    if (!classid) return E_POINTER;
+
+    *classid = CLSID_XMLDocument;
+    return S_OK;
 }
 
 static HRESULT WINAPI xmldoc_IPersistStreamInit_IsDirty(