Bug 1615370 - Add fallback to ShellExecuteExW in nsMIMEInfoWin. r=aklotz

If a system uses a custom shell instead of Windows Explorer,
`ShellExecuteByExplorer` always fails because it relies on explorer.exe,
but `ShellExecute` stil works because it's in-proc implementation.

We added a fallback for a local file to address Bug 1602726.  This patch adds
a fallback for a uri.

Differential Revision: https://phabricator.services.mozilla.com/D63095

--HG--
extra : moz-landing-system : lando

uriloader/exthandler/win/nsMIMEInfoWin.cpp

@@ -283,10 +283,19 @@ nsresult nsMIMEInfoWin::LoadUriInternal(nsIURI* aURL) {
     // Ask Explorer to ShellExecute on our behalf, as some URL handlers do not
     // start correctly when inheriting our process's process migitations.
     mozilla::LauncherVoidResult shellExecuteOk =
-        mozilla::ShellExecuteByExplorer(validatedUri.unwrap(), args, verb,
+        mozilla::ShellExecuteByExplorer(validatedUri.inspect(), args, verb,
                                         workingDir, showCmd);
     if (shellExecuteOk.isErr()) {
-      return NS_ERROR_FAILURE;
+      SHELLEXECUTEINFOW sinfo = {sizeof(sinfo)};
+      sinfo.fMask = SEE_MASK_NOASYNC;
+      sinfo.lpVerb = V_BSTR(&verb);
+      sinfo.nShow = showCmd;
+      sinfo.lpFile = validatedUri.inspect();
+
+      BOOL result = ShellExecuteExW(&sinfo);
+      if (!result || reinterpret_cast<LONG_PTR>(sinfo.hInstApp) < 32) {
+        rv = NS_ERROR_FAILURE;
+      }
     }
   }