Implement fetching, caching and retrieval of external policy data

chrome/app/policy/policy_templates.json

 {
 # policy_templates.json - Metafile for policy templates
 #
 # The content of this file is evaluated as a Python expression.
 #
 # This file is used as input to generate the following policy templates:
 # ADM, ADMX+ADML, MCX/plist and html documentation.
 #
 # Policy templates are user interface definitions or documents about the
 # policies that can be used to configure Chrome. Each policy is a name-value
@@ skipping 12 matching lines @@
 #   'string' - a string value
 #   'int' - an integer value
 #   'int-enum' - the user can select an integer value from a collection of
 #     items
 #   'string-enum' - the user can select a string value from a collection of
 #     items
 #   'main' - a boolean value
 #   'list' - a list of string values
 #   'dict' - a dictionary value, containing other values indexed by strings
 #     NOTE: This type is not supported yet. http://crbug.com/108992
+#   'external' - a policy that references external data.
+#     NOTE: This type is not supported yet. http://crbug.com/256635
 #
 # Policy group descriptions, policy captions and similar texts are localized
 # strings taken from the <message> nodes of the .grd file. Their name
 # attributes are generated from the JSON keys.
 # Each item (policy or group) may have the following messages:
 #   - description:
 #     Describes the item it applies to.
 #   - caption
 #     A short, one-line summary of the item it applies to. This can appear
 #     both in policy or group listings or on title bars of policy-setting
@@ skipping 5261 matching lines @@
       'desc': '''Text appended in parentheses to the policy name to indicate that it has been deprecated''',
       'text': 'deprecated',
     },
     'doc_recommended': {
       'desc': '''Text appended in parentheses next to the policies top-level container to indicate that those policies are of the Recommended level''',
       'text': 'Default Settings (users can override)',
     },
   },
   'placeholders': [],
 }