Refactor Content-Range response header parsing into http_util

Refactor Content-Range response header parsing into http_util so that
it's easier for other components to depend on it without needing to go
through HttpResponseHeaders.

BUG=670913

Review-Url: https://codereview.chromium.org/2545213002
Cr-Commit-Position: refs/heads/master@{#436162}
4 files changed