This CL moves proxy_config_dictionary and proxy_prefs to
src/components/proxy_config so that they can be used in
src/chromeos/network/onc for ONC translation.

BUG=470445
TBR=various for moving include files

Review URL: https://codereview.chromium.org/1221033002

Cr-Commit-Position: refs/heads/master@{#337257}