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/188740Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>