Add ONC 'Source' configuration property

This also includes a bit of JS cleanup to use ONC properties directly
where possible.

BUG=279351
[email protected]

Review URL: https://codereview.chromium.org/552113002

Cr-Commit-Position: refs/heads/master@{#294462}
diff --git a/components/onc/onc_constants.h b/components/onc/onc_constants.h
index 0029ca7d..14ddb98 100644
--- a/components/onc/onc_constants.h
+++ b/components/onc/onc_constants.h
@@ -13,6 +13,7 @@
 
 // Indicates from which source an ONC blob comes from.
 enum ONCSource {
+  ONC_SOURCE_UNKNOWN,
   ONC_SOURCE_NONE,
   ONC_SOURCE_USER_IMPORT,
   ONC_SOURCE_DEVICE_POLICY,
@@ -69,6 +70,12 @@
 ONC_EXPORT extern const char kPriority[];
 ONC_EXPORT extern const char kProxySettings[];
 ONC_EXPORT extern const char kSearchDomains[];
+ONC_EXPORT extern const char kSource[];
+ONC_EXPORT extern const char kSourceDevice[];
+ONC_EXPORT extern const char kSourceDevicePolicy[];
+ONC_EXPORT extern const char kSourceNone[];
+ONC_EXPORT extern const char kSourceUser[];
+ONC_EXPORT extern const char kSourceUserPolicy[];
 ONC_EXPORT extern const char kConnectionState[];
 ONC_EXPORT extern const char kRestrictedConnectivity[];
 ONC_EXPORT extern const char kConnectable[];