Remove confusing DLOG from SingletonHwnd

Sheriffs and general bug fixers often find this logspam confusing,
thinking it relevant to the problem at hand. However it's rare that the
actual perpetrator sees it when adding the problem. So keep the blocking
behaviour, but remove the log.

R=thakis@chromium.org
BUG=258201

Review-Url: https://codereview.chromium.org/2286943002
Cr-Commit-Position: refs/heads/master@{#414824}

ui/gfx/win/singleton_hwnd.cc

@@ -31,7 +31,6 @@ SingletonHwnd::SingletonHwnd() {
   if (!base::MessageLoopForUI::IsCurrent()) {
     // Creating this window in (e.g.) a renderer inhibits shutdown on
     // Windows. See http://crbug.com/230122 and http://crbug.com/236039.
-    DLOG(ERROR) << "Cannot create windows on non-UI thread!";
     return;
   }
   WindowImpl::Init(NULL, Rect());