wifi_sync: add ability to convert WifiCredential to onc properties
There will be a couple of places where we need to convert from
a WifiCredential, to a DictionaryValue of ONC properties. So let's
put the conversion code someplace where it can be re-used.
The two places we'll need to convert a WifiCredential to ONC
properties will be WifiConfigDelegate and sync integration tests.
Both will need ONC properties to create a network in the
platform-specific network configuration for ChromeOS.
BUG=chromium:431435
TEST=components_unittests --gtest_filter="Wifi*"
Review URL: https://codereview.chromium.org/
809803005
Cr-Commit-Position: refs/heads/master@{#310930}