Use FileTypePolicies for download danger classifications.

This removes download_extensions.{h,cc}

BUG=596555
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/1982723002
Cr-Commit-Position: refs/heads/master@{#394914}
21 files changed