GN change disallows sources that aren't a source, header, or object file.
We need to pass `process_only` flag to process_version to not create source_set
out of the version file.

GN change:

https://gn.googlesource.com/gn/+/8730b0feb6b991fa47368566501ab9ccfb453c92

commit	8730b0feb6b991fa47368566501ab9ccfb453c92
author	Tom Anderson <thomasanderson@chromium.org>	Thu May 16 11:00:40 2019 -0700
committer	Commit Bot <commit-bot@chromium.org>	Thu May 16 21:36:50 2019 +0000

Disallow non-buildable sources in binary targets

When adding a file that's not a source, header, or object file to a source_set,
loadable_module, shared_library, executable, or static_library, gn will now
generate an error like the following:

ERROR at //third_party/protobuf/proto_library.gni:369:15: Only source, header,
and object files belong in the sources of a
source_set. //out/Test/pyproto/google_apis/gcm/protocol/mcs_pb2.py is not one of
the valid types.
    sources = get_target_outputs(":$action_name")
              ^---------------------------------
See //google_apis/gcm/BUILD.gn:78:1: whence it was called.
proto_library("proto") {
^-----------------------
See //BUILD.gn:89:7: which caused the file to be included.
      "//google_apis/gcm:gcm_unit_tests",
      ^---------------------------------

BUG=77