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

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.

BUG=596555

Review URL: https://codereview.chromium.org/1857983002

Cr-Commit-Position: refs/heads/master@{#386818}
10 files changed