Fix crash in SelectFileDialogImplGTK::OnFileChooserDestroyThunk.

Here is the call stack when the crash happens:
(chrome - scoped_refptr.h: 191)	libgtkui::SelectFileDialogImplGTK::OnFileChooserDestroyThunk(_GtkWidget*, void*)
(chrome - select_file_dialog_impl_gtk.cc: 98)	libgtkui::SelectFileDialogImplGTK::~SelectFileDialogImplGTK()
(chrome - select_file_dialog_impl_gtk.cc: 89)	<name omitted>
(chrome - ref_counted.h: 414)	DownloadFilePicker::~DownloadFilePicker()
(chrome - select_file_dialog_impl_gtk.cc: 284)	libgtkui::SelectFileDialogImplGTK::FileSelected(_GtkWidget*, base::FilePath const&)

A dialog is destroyed in FileSelected(), but it seems that the same
dialog is destroyed again in OnFileChooserDestroy.

BUG=880073

Change-Id: I1f47bbc26b4af81b4c73d7cf03ac684edc4f97fd
Reviewed-on: https://chromium-review.googlesource.com/1256020
Commit-Queue: Joone Hur <joone.hur@intel.com>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595612}

chrome/browser/ui/libgtkui/select_file_dialog_impl_gtk.cc

@@ -532,6 +532,11 @@ void SelectFileDialogImplGTK::OnSelectMultiFileDialogResponse(GtkWidget* dialog,
 }
 
 void SelectFileDialogImplGTK::OnFileChooserDestroy(GtkWidget* dialog) {
+  // There might be a case that |dialog| is already deleted
+  // in SelectFileDialogImplGTK::FileSelected.
+  // See https://crbug.com/880073.
+  if (!dialog)
+    return;
   // |parent| can be nullptr when closing the host window
   // while opening the file-picker.
   aura::Window* parent = dialogs_[dialog]->parent;