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 49 matching lines @@
 # Annotations:
 #   Additional information is specified under keys 'features' and
 #   'example_value'. These are used in the generated documentation and example
 #   policy configuration files. 'dynamic_refresh' controls if the generated
 #   documentation should state that the policy supports dynamic refresh or not.
 #   Supporting dynamic refresh means that Chrome respects the changes to the
 #   policy immediately, without the need for restart.
 #   'can_be_recommended' can be set to True to include that policy in the
 #   recommended policies templates. This only affects the template generation;
 #   all policies can be at the recommended level. The default is False.
+#   'max_size' specifies the maximal size of the external data that a policy can
+#   reference, in bytes. This annotation is compulsory for policies of type
+#   'external'. It is ignored for all other policy types.
 #
 #   The 'future' key is used to indicate that a policy isn't yet ready for
 #   usage. It defaults to False, and currently affects the generated policy
 #   templates and documentation. The policy definition list that Chrome sees
 #   will include policies marked with 'future'. If a WIP policy isn't meant to
 #   be seen by the policy providers either, the 'supported_on' key should be set
 #   to an empty list.
 #
 # IDs:
 #   Since a Protocol Buffer definition is generated from this file, unique and
@@ skipping 5396 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': [],
 }