Move DnsConfig to separate file.

Moving to a new dns_config.cc/h to separate the public struct from the
component-private DnsConfigService. Creates a better place for a new
DnsConfigOverrides struct that I'll be adding soon for the servicified
DNS config manipulation work.

[email protected]

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;master.tryserver.chromium.android:android_cronet_tester
Change-Id: I5e02cc29b47851ea5295d2ec7d17b381b124d97a
Reviewed-on: https://chromium-review.googlesource.com/1255783
Commit-Queue: Eric Orth <[email protected]>
Reviewed-by: Matt Menke <[email protected]>
Cr-Commit-Position: refs/heads/master@{#595839}
31 files changed