Bug 1701794 - Use nsOSHelperAppServiceChild with Windows r=nika,haik

Currently, the Windows implemenation of nsOSHelperAppService uses COM to figure out MIME types and extension
associations. That is incompatible with Win32k lockdown.

Luckily, Haik already made a class that handles these calls in content process and either marks them as
unimplemented or-else forwards them to the main process. This seems to work fine on Windows, so it looks like all
that needs to be done is adding XP_WIN to a couple of #ifdefs and we're good to go!

Differential Revision: https://phabricator.services.mozilla.com/D118391

uriloader/exthandler/HandlerServiceParent.cpp

@@ -321,7 +321,7 @@ mozilla::ipc::IPCResult HandlerServiceParent::RecvExistsForProtocol(
     *aHandlerExists = false;
     return IPC_OK();
   }
-#if defined(XP_MACOSX)
+#if defined(XP_MACOSX) || defined(XP_WIN)
   // Check the datastore and fallback to an OS check.
   // ExternalProcotolHandlerExists() does the fallback.
   nsresult rv;

uriloader/exthandler/nsExternalHelperAppService.cpp

@@ -638,7 +638,7 @@ static StaticRefPtr<nsExternalHelperAppService> sExtHelperAppSvcSingleton;
 already_AddRefed<nsExternalHelperAppService>
 nsExternalHelperAppService::GetSingleton() {
   if (!sExtHelperAppSvcSingleton) {
-#ifdef XP_MACOSX
+#if defined(XP_MACOSX) || defined(XP_WIN)
     if (XRE_IsParentProcess()) {
       sExtHelperAppSvcSingleton = new nsOSHelperAppService();
     } else {
@@ -646,7 +646,7 @@ nsExternalHelperAppService::GetSingleton() {
     }
 #else
     sExtHelperAppSvcSingleton = new nsOSHelperAppService();
-#endif /* XP_MACOSX */
+#endif  // defined(XP_MACOSX) || defined(XP_WIN)
     ClearOnShutdown(&sExtHelperAppSvcSingleton);
   }