Add a policy to disable mounting of external storage.

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 94 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: 150
+#   For your editing convenience: highest ID currently used: 151
 #
 # 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 2619 matching lines @@
       'features': {'dynamic_refresh': True},
       'example_value': True,
       'id': 150,
       'caption': '''Disable proceeding from the Safe Browsing warning page''',
       'desc': '''Disable proceeding from the Safe Browsing warning page.
 
       The Safe Browsing service shows a warning page when users navigate to sites that are flagged as potentially mallicious. Enabling this setting prevents users from proceeding anyway from the warning page to the malicious site.
 
       If this setting is disabled or not configured then users can choose to proceed to the flagged site after being shown the warning.''',
     },
+    {
+      'name': 'ExternalStorageDisabled',
+      'type': 'main',
+      'supported_on': ['chrome_os:0.22-'],
+      'features': {'dynamic_refresh': True},
+      'example_value': True,
+      'id': 151,
+      'caption': '''Disable mounting of external storage''',
+      'desc': '''Disable mounting of external storage.
+
+      When this policy is set to true, external storage will not be available in the file browser.
+
+      This policy affects all types of storage media. For example: USB flash drives, external hard drives, SD and other memory cards, optical storage etc.

[Mattias Nissler (ping if slow), 2012/07/17 08:26:30]

nit: either join the lines are put a blank line for paragraph breaking (which is
the de-facto style found in this file).

+      Internal storage is not affected, therefore files saved in the Download folder can still be accessed. Google Drive is also not affected by this policy.
+
+      If this setting is disabled or not configured then users can use all supported types of external storage on their device.''',
+    },
   ],
   '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 78 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': 'Recommended',
     },
   },
   'placeholders': [],
 }