cefclient: Add option to create default popups (see issue #3165, see issue #3294)

This change adds a `--use-default-popup` command-line option to cefclient. When
specified, popup windows will be created with default styling (e.g. without an
application-provided native parent window).

This change also adds some reasonable default window bounds in cases where they
are not specified by the client.

libcef/browser/native/browser_platform_delegate_native_win.cc

@@ -178,6 +178,11 @@ void CefBrowserPlatformDelegateNativeWin::BrowserDestroyed(
 bool CefBrowserPlatformDelegateNativeWin::CreateHostWindow() {
   RegisterWindowClass();
 
+  if (window_info_.style == 0) {
+    // Client didn't intialize the CefWindowInfo. Provide reasonable defaults.
+    window_info_.SetAsPopup(nullptr, CefString());
+  }
+
   has_frame_ = !(window_info_.style & WS_CHILD);
 
   std::wstring windowName(CefString(&window_info_.window_name));