Generate WKT for protobuf on the fly in CMake

Chromium has made changes to their version of protobuf, which Dawn
depends on, to dynamically generate well known types (WKT) to resolve
issues with staleness between the .proto and .pb.* files. Since
Chromium only supports GN builds, this change was not reflected in an
upstream patch for CMake builds.

Unfortunately due to libprotobuf-mutator depending on protobuf, and
how CMake differs from GN, there isn't a simple patch to be added to
the upstream stack to address this. It would require an equivalent
patch to lpm, and as-is Chromium doesn't carry patches against lpm.

Given Chromium doesn't maintain CMake support, asking them to start
carrying patches against another library feels like a big request. So
instead this CL introduces a less invasive change to rewrite target
usages and build the missing files on the fly.

Fixes: 514654755
Change-Id: I68e3ba9a5a2ff97babe1426e185c2accfc181b50
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/309595
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>