Was mistakenly returning a boolean instead of EGLint.

BUG=angle:850

Change-Id: I07f0995d66783f62124d37f5ea8e595f516cc84f
Reviewed-on: https://chromium-review.googlesource.com/244844Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>