Add policy provider that would filter extensions/apps allowed on the

extensions/common/api/_behavior_features.json

Index: extensions/common/api/_behavior_features.json
diff --git a/extensions/common/api/_behavior_features.json b/extensions/common/api/_behavior_features.json
index 73dab5138f9ed6bbddbfd3aa481000377c20633f..d2b553370dc0a859ed999c4a172ce4d1a15518d4 100644
--- a/extensions/common/api/_behavior_features.json
+++ b/extensions/common/api/_behavior_features.json
@@ -46,5 +46,42 @@
     "channel": "dev",
     "extension_types": ["platform_app"],
     "session_types": ["kiosk"] 
+  },
+  "signin_screen": [ {
+    "blacklist": [
+      "9E527CDA9D7C50844E8A5DB964A54A640AE48F98"   // Chrome remote desktop
+    ],
+    // Allow developers to test new features.
+    "channel": "dev",

[Devlin, 2016/11/11 16:35:08]

Note: we have seen malware that modifies the perceived channel of chrome.  Is
this a security risk?

[asargent_no_longer_on_chrome, 2016/11/11 17:37:13]

Given that this the platforms field only contains chromeos here, I don't think
we need to worry. (I don't think there even is a signin screen on non-chromeos ;
the idea behind this CL is to limit the kinds of extensions that can run on the
signin screen)

[Denis Kuznetsov (DE-MUC), 2016/11/11 21:26:34]

Right. And there are extra checks - type of the extension, installation
location, etc.
There will be more checks (e.g. requested permissions) in upcoming CLs.

+    "component_extensions_auto_granted": false,
+    "extension_types": ["platform_app"],
+    "location": "policy",
+    "platforms": ["chromeos"]
+  }, {
+    // Strict restrictions for stable channel.
+    // For now only whitelisted apps.
+    "channel": "stable",
+    "component_extensions_auto_granted": false,
+    "extension_types": ["platform_app"],
+    "location": "policy",
+    "platforms": ["chromeos"],
+    "whitelist": [
+      "EC3DE21E048B67319893889529354DFBFA96FD23",  // Smart Card Connector
+      "6B748A5C005F21B7CBCF4170C2F883E435DEB511"   // CSSI Smart Card Middleware
+    ]
+  }, {
+    // Explicitly whitelist extensions that should always be available
+    // on a sign-in screen.
+    "channel": "stable",
+    "component_extensions_auto_granted": false,
+    "platforms": ["chromeos"],
+    "location": "component",
+    "whitelist": [
+      "E24F1786D842E91E74C27929B0B3715A4689A473",  // Gnubby component extension
+      "6F9E349A0561C78A0D3F41496FE521C5151C7F71",  // Gnubby app
+      "8EBDF73405D0B84CEABB8C7513C9B9FA9F1DC2CE",  // Genius app (help)
+      "3F50C3A83839D9C76334BCE81CDEC06174F266AF"   // Virtual Keyboard
+    ]
   }
+  ]
 }