Introduce session type parameter to extension features

chrome/common/extensions/api/_features.md

 # Extension Features Files
 
 [TOC]
 
 ## Summary
 
 The Extension features files specify the different requirements for extension
 feature availability.
 
 An **extension feature** can be any component of extension capabilities. Most
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 The only accepted value is the bool `true`.
 
 ### platforms
 
 The `platforms` property specifies the properties the feature should be
 available on.
 
 The accepted values are lists of strings from `chromeos`, `mac`, `linux`, and
 `win`.
 
+### session\_types
+
+The `session_types` property specifies in which types of sessions a feature
+should be available. The session type describes the type of user that is
+logged in the current session. Session types to which feature can be restricted
+are only supported on Chrome OS - features restricted to set of session types
+will be disabled on other platforms. Also, note that all currently supported
+session types imply that a user is logged into the session (i.e. features that
+use 'session_types' property will be disabled when a user is not logged in).
+
+The accepted values are lists of strings from `regular` and `kiosk`.
+
 ### whitelist
 
 The `whitelist` property specifies a list of ID hashes for extensions that
 are the only extensions allowed to access a feature.
 
 Accepted values are lists of id hashes.
 
 ## ID Hashes
 
 Instead of listing the ID directly in the whitelist or blacklist section, we
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 at runtime, this has the added benefit of allowing us to validate at compile
 time rather than needing a unittest (or allowing incorrect features).
 
 In theory, invalid features should result in a compilation failure; in practice,
 the compiler is probably missing some cases.
 
 ## Still to come
 
 TODO(devlin): Add documentation for extension types. Probably also more on
 requirements for individual features.