Moves settings model and related classes to code common for both iOS
and Mac.
BUG=webrtc:7177,webrtc:6494
Review-Url: https://codereview.webrtc.org/2770113004
Cr-Commit-Position: refs/heads/master@{#17408}
This enables tighter integration with XCode tooling and is a prereq
for adding UI tests.
BUG=webrtc:7150
Review-Url: https://codereview.webrtc.org/2697603002
Cr-Commit-Position: refs/heads/master@{#16609}