combase: Implement RoActivateInstance.

dlls/api-ms-win-core-winrt-l1-1-0/api-ms-win-core-winrt-l1-1-0.spec

-@ stub RoActivateInstance
+@ stdcall RoActivateInstance(ptr ptr) combase.RoActivateInstance
 @ stdcall RoGetActivationFactory(ptr ptr ptr) combase.RoGetActivationFactory
 @ stub RoGetApartmentIdentifier
 @ stdcall RoInitialize(long) combase.RoInitialize

dlls/combase/combase.spec

@@ -242,7 +242,7 @@
 @ stdcall PropVariantClear(ptr) ole32.PropVariantClear
 @ stdcall PropVariantCopy(ptr ptr) ole32.PropVariantCopy
 @ stub ReleaseFuncDescs
-@ stub RoActivateInstance
+@ stdcall RoActivateInstance(ptr ptr)
 @ stub RoCaptureErrorContext
 @ stub RoClearError
 @ stub RoFailFastWithErrorContext

dlls/combase/roapi.c

@@ -198,3 +198,23 @@ HRESULT WINAPI RoGetParameterizedTypeInstanceIID(UINT32 name_element_count, cons
     if (hiid) *hiid = INVALID_HANDLE_VALUE;
     return E_NOTIMPL;
 }
+
+/***********************************************************************
+ *      RoActivateInstance (combase.@)
+ */
+HRESULT WINAPI RoActivateInstance(HSTRING classid, IInspectable **instance)
+{
+    IActivationFactory *factory;
+    HRESULT hr;
+
+    FIXME("(%p, %p): semi-stub\n", classid, instance);
+
+    hr = RoGetActivationFactory(classid, &IID_IActivationFactory, (void **)&factory);
+    if (SUCCEEDED(hr))
+    {
+        hr = IActivationFactory_ActivateInstance(factory, instance);
+        IActivationFactory_Release(factory);
+    }
+
+    return hr;
+}