Fixes crash of SetGtkTransientForAura.

The crash report is saying that the crash still happens in Aura.
I still don't get the repro steps, but on possibility is that
the parent exists but WindowTreeHost doesn't exist (missing
root windows? I don't even know that is possible though...).

BUG=373522
R=erg@chromium.org
TEST=none

Review URL: https://codereview.chromium.org/304223012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274341 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/ui/libgtk2ui/gtk2_util.cc

@@ -104,7 +104,7 @@ int EventFlagsFromGdkState(guint state) {
 }
 
 void SetGtkTransientForAura(GtkWidget* dialog, aura::Window* parent) {
-  if (!parent)
+  if (!parent || !parent->GetHost())
     return;
 
   gtk_widget_realize(dialog);