Remove net::Filter code

This CL removes net::Filter code since the new net::SourceStream API is
integrated.

This CL additionally cleans up SDCH UMAs that no longer exist in the codebase.

BUG=474859

Review-Url: https://codereview.chromium.org/2478703004
Cr-Commit-Position: refs/heads/master@{#431302}
18 files changed