Use packed enums on shader types in ANGLE renderer 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>