Move the Validation checks for SwapBuffers into new ValidateSwapBuffers
function instead of in-line.

This is a first commit for refactor of EGL validation. I'll continue
the refactor in a similar fashion to this.

Bug: angleproject:798
Change-Id: Icb308d66c93528c9e440ae68abe85e44f98ede82
Reviewed-on: https://chromium-review.googlesource.com/1151538
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>