net: remove GetNormalizedHeaders() function

It is only used by some logging and test functions. It should be safe to
replace those callers with ConvertHeadersBackToHTTPResponse().

This cleans up Darin's TODO.

BUG=488593
TEST=net_unittests
R=eroman@chromium.org,rogerta@chromium.org,sky@chromium.org

Review-Url: https://codereview.chromium.org/2670013002
Cr-Commit-Position: refs/heads/master@{#451455}

• content/public/test/test_download_request_handler.cc[diff]
• google_apis/drive/base_requests.cc[diff]
• google_apis/gaia/gaia_auth_fetcher.cc[diff]
• net/http/http_cache_unittest.cc[diff]
• net/http/http_response_headers.cc[diff]
• net/http/http_response_headers.h[diff]