Add generator for packed GL enums.

For testing this also converts two unimportant GLenums, gl::BufferUsage
and gl::CullModeFace.

BUG=angleproject:2169

Change-Id: If1e86a97d0fed3fd567303aca6506ec579503076
Reviewed-on: https://chromium-review.googlesource.com/688000
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 168d8a0..15274e4 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -3595,7 +3595,7 @@
     mGLState.setColorMask(red == GL_TRUE, green == GL_TRUE, blue == GL_TRUE, alpha == GL_TRUE);
 }
 
-void Context::cullFace(GLenum mode)
+void Context::cullFace(CullFaceMode mode)
 {
     mGLState.setCullMode(mode);
 }
@@ -4001,7 +4001,7 @@
     mGLState.getDebug().popGroup();
 }
 
-void Context::bufferData(GLenum target, GLsizeiptr size, const void *data, GLenum usage)
+void Context::bufferData(GLenum target, GLsizeiptr size, const void *data, BufferUsage usage)
 {
     Buffer *buffer = mGLState.getTargetBuffer(target);
     ASSERT(buffer);