Add a policy to re-enable deprecated web platform features.

components/policy/resources/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 100 matching lines @@
 #   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
 #   persistent IDs for all fields (but not for groups!) are needed. These are
 #   specified by the 'id' keys of each policy. NEVER CHANGE EXISTING IDs,
 #   because doing so would break the deployed wire format!
-#   For your editing convenience: highest ID currently used: 264
+#   For your editing convenience: highest ID currently used: 265
 #
 # Placeholders:
 #   The following placeholder strings are automatically substituted:
 #     $1 -> Google Chrome / Chromium
 #     $2 -> Google Chrome OS / Chromium OS
 #     $3 -> Google Chrome Frame / Chromium Frame
 #     $6 is reserved for doc_writer
 #
 # Device Policy:
 #   An additional flag device_only (optional, defaults to False) indicates
@@ skipping 6226 matching lines @@
           }
         }
       }
 
       If this policy is set, <ph name="PRODUCT_OS_NAME">$2<ex>Google Chrome OS</ex></ph> will download and use the wallpaper image.
 
       If you set this policy, users cannot change or override it.
 
       If the policy is left not set, the user can choose an image to be shown on the desktop and on the login screen background.''',
     },
+    {
+      'name': 'EnableDeprecatedWebPlatformFeatures',
+      'type': 'list',
+      'schema': {
+        'type': 'array',
+        'items': { 'type': 'string' },
+      },
+      'supported_on': ['chrome.*:36-', 'chrome_os:36-'],
+      'features': {
+        'dynamic_refresh': True,
+        'per_profile': True,
+      },
+      'example_value': [],
+      'id': 265,
+      'caption': '''Enable deprecated web platform features''',
+      'desc': '''Specify a list of deprecated web platform features to re-enable.
+
+      This policy gives administrators the ability to re-enable deprecated web platform features for a limited time. Features are identified by a string tag and the features corresponding to the tags included in the list specified by this policy will get re-enabled.
+
+      The following tags are currently defined:
+      - ShowModalDialog

[Andrew T Wilson (Slow), 2014/05/08 11:32:45]

Should update this to include an expiration date as we previously discussed

[abarth-chromium, 2014/05/10 00:27:39]

Yeah, I think that's a good idea to make sure admins know how long they've got.

[Mattias Nissler (ping if slow), 2014/05/19 15:03:08]

Done.

+
+      If this policy is left not set or the list is empty, all deprecated web platform features will remain disabled.''',
+    },
   ],
   'messages': {
     # Messages that are not associated to any policies.
     'win_supported_winxpsp2': {
       'desc': '''A label specifying the oldest possible compatible version of Windows. This text will appear right next to a label containing the text 'Supported on:'.''',
       'text': '''Microsoft Windows XP SP2 or later'''
     },
     'mac_chrome_preferences': {
       'desc': '''A text indicating in Mac OS X Workgroup Manager, that currently the preferences of Chrome are being edited''',
       'text': '''<ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> preferences'''
@@ skipping 88 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': [],
 }