Revert "network service: define an opaque ResourceType enum for embedder customization"

This reverts commit da5a52f67a28b32a0384d8c2253b5ef98f28d74f.

Reason for revert: Decided not to use the trick of declaring an opaque enum,
since it's potentially confusing. Once network service is fully enabled, it
won't be needed as ResourceType won't need to be in the network service at all.

Original change's description:
> network service: define an opaque ResourceType enum for embedder customization
>
> Network service embedders that use the resource_type field can provide
> a custom definition of the enum to suit their needs.
>
> Tbr: jochen@chromium.org
> Change-Id: I27c04257c90e8cf0353287b7ec8971593772dac8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1575519
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Reviewed-by: Matt Menke <mmenke@chromium.org>
> Reviewed-by: Bo <boliu@chromium.org>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#653476}

TBR=avi@chromium.org,dcheng@chromium.org,thestig@chromium.org,boliu@chromium.org,mmenke@chromium.org,tsepez@chromium.org,jochen@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Tbr: jam@chromium.org
Change-Id: Ib91a35bea1225c04bbf25f7c6b82e17b9b59dba8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597220
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657157}
62 files changed