Bug 1592444 - Allow E_ACCESSDENIED from CoAllowSetForegroundWindow in debug build. r=aklotz

Bug 1588500 reported some of Thunderbird's tests failed because
`CoAllowSetForegroundWindow` failed.  In that case, those tests should not launch
a default browser of the system, but we should allow `E_ACCESSDENIED` because
it legitimately happens if the window is not in the foreground.

Differential Revision: https://phabricator.services.mozilla.com/D52618

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

widget/windows/ShellHeaderOnlyUtils.h

@@ -126,10 +126,11 @@ inline LauncherVoidResult ShellExecuteByExplorer(const _bstr_t& aPath,
   }
 
   // Passing the foreground privilege so that the shell can launch an
-  // application in the foreground.  If CoAllowSetForegroundWindow fails,
-  // we continue because it's not fatal.
+  // application in the foreground.  This fails with E_ACCESSDENIED if the
+  // current window is shown in the background.  We keep a soft assert for
+  // the other failures to investigate how it happened.
   hr = ::CoAllowSetForegroundWindow(shellDisp, nullptr);
-  MOZ_ASSERT(SUCCEEDED(hr));
+  MOZ_ASSERT(SUCCEEDED(hr) || hr == E_ACCESSDENIED);
 
   // shellapi.h macros interfere with the correct naming of the method being
   // called on IShellDispatch2. Temporarily remove that definition.