WindowSurfaceGLX: Fix child window creation size This makes Chrome on ANGLE work on Linux without obvious bugs. BUG=angleproject:892 Change-Id: I595ac00a6646cb407941932b0d4f9f020d4cd059 Reviewed-on: https://chromium-review.googlesource.com/282057 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>