Introduce session type parameter to extension features

This adds session type to extension features grammar. The session
type refers to the type of user active in the session in which
an extension is running. The parameter will enable making a feature
available depending on type of the session in which the extension/app
is running.

Feature session types are declared as list of individual session types
in which a feature is available (undefined list means no restrictions).
Currently, values supported for session type are as follows:
* 'kiosk' - kiosk app session
* 'regular' -  non-kiosk user session
The list of supported values is expected to be expanded in future.

Depends on https://codereview.chromium.org/2241203003/ for plumbing
of current session type to feature availability checks

BUG=606417

Review-Url: https://codereview.chromium.org/2255613003
Cr-Commit-Position: refs/heads/master@{#418938}
diff --git a/chrome/common/extensions/api/_features.md b/chrome/common/extensions/api/_features.md
index baad63aa..bd4a86e 100644
--- a/chrome/common/extensions/api/_features.md
+++ b/chrome/common/extensions/api/_features.md
@@ -253,6 +253,18 @@
 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