This patch uses a packed internal enum ShaderType everywhere we
need a shader type instead of the GLenum value of the shader type.

This patch also uses program::getAttachedShader(type) everywhere
we need to get gl::Shader from a program in ANGLE.

BUG=angleproject:2169
Change-Id: I28a7fa1cfe35622c57a486932911110688eaadec
Reviewed-on: https://chromium-review.googlesource.com/972844
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>