Re-land: Add download_file_types.proto with ascii->binary conversion, as a resource.

This is a revert of the revert

Added: Missing Python path, on some bots.

Orig desc: We'll maintain an ASCII version of the data in the repo, and build
the binary version as a Resource at build time.  The data will later be
updated at runtime.


Review URL:

Cr-Commit-Position: refs/heads/master@{#387109}
10 files changed