Port the PostSubBuffer sample to the new sample framework. Add a swap method on the SampleApplication class so that samples can override the default eglSwapBuffers function call. BUG=angle:521 Change-Id: I3f0ba6ed182b368e6f5fe502de67d1a14267a857 Reviewed-on: https://chromium-review.googlesource.com/188740 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>