FunctionsEGL is an abstract class.
FunctionsEGLDL is an libdl-based implementation.

BUG=angleproject:1297

Change-Id: I0fe5a337e34518cc6248494dbc4ee5c2938d4b77
Reviewed-on: https://chromium-review.googlesource.com/338884Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>