Introduce session type parameter to extension features

chrome/common/extensions/api/_features.md

Index: chrome/common/extensions/api/_features.md
diff --git a/chrome/common/extensions/api/_features.md b/chrome/common/extensions/api/_features.md
index baad63aa5d952ec41caa8548a307fdb8f892c828..1f3acf45acb6363f59c20e18a3f7b071a74e9811 100644
--- a/chrome/common/extensions/api/_features.md
+++ b/chrome/common/extensions/api/_features.md
@@ -253,6 +253,17 @@ 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 - feature restricted to set of session types

[Devlin, 2016/09/13 17:37:45]

nit: s/features/feature

[tbarzic, 2016/09/13 17:55:12]

Done.

+will be disabled on other platforms. Also, note that all currently supported
+session types imply that a user is logged into the session.

[Devlin, 2016/09/13 17:37:45]

Mention what the value is when the user is *not* logged in?

Also, this still makes me a little nervous, because it means things like the
following break:
feature: {
  session_types: [regular]
}
feature.child: {
  session_types: [???]  // How do I get all session types?
}

I'm okay addressing that at a later date, but keep it in mind.

[tbarzic, 2016/09/13 17:55:12]

That one is not currently supported (as there is no need to enable an API only
when a user is not logged in).
(except for handful of exceptions (Chromevox and easy unlock app, afaik),
extensions are not allowed before a user is logged in on Chrome OS)
If this has to be supported, we could add another enum value 'all', or maybe set
the value to session_types: <all> (this could apply to other feature properties,
e.g. platforms)

[Devlin, 2016/09/13 18:13:50]

I understand we don't offer it as a value, but it's important to note that if a
user is not logged in, it will not be considered any of the session types.  This
is somewhat implied by the last sentence already, but I'd like it to be
explicit.
Many properties support all - e.g. contexts.  The difference is that "all" for
something like platforms just encompasses the platforms that can be specified
already - i.e., I can specify win, linux, chromeos, and mac.  For session types,
that's not the case.  And I think it's very confusing for 'all' to imply more
than the collection of available enums.

[tbarzic, 2016/09/13 18:25:40]

Done.
Ack

+
+The accepted values are lists of strings from `regular` and `kiosk`.
+
 ### whitelist
 
 The `whitelist` property specifies a list of ID hashes for extensions that