Add ExternalStorageReadOnly profile value.

components/policy/resources/policy_templates.json

Index: components/policy/resources/policy_templates.json
diff --git a/components/policy/resources/policy_templates.json b/components/policy/resources/policy_templates.json
index 51b69a57ba9ec6d856bb8f12fb03b5da466d04f0..71ba1845825d856c12a133511fb426193c8e9411 100644
--- a/components/policy/resources/policy_templates.json
+++ b/components/policy/resources/policy_templates.json
@@ -137,7 +137,7 @@
 #   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: 342
+#   For your editing convenience: highest ID currently used: 343
 #
 # Placeholders:
 #   The following placeholder strings are automatically substituted:
@@ -5625,6 +5625,21 @@
       If this setting is disabled or not configured then users can use all supported types of external storage on their device.''',
     },
     {
+      'name': 'ExternalStorageReadOnly',
+      'id': 343,
+      'type': 'main',
+      'schema': { 'type': 'boolean' },
+      'tags': [],
+      'supported_on': ['chrome_os:54-'],
+      'caption': '''Treat external storage devices as read-only.''',
+      'desc': '''When this policy is set to true, users cannot write anything to external storage devices.''',
+      'features': {
+        'dynamic_refresh': False,

[bartfab (slow), 2016/08/29 14:50:40]

Why can we not change this at runtime? We try really hard to support dynamic
refresh for all policies.

[yamaguchi, 2016/08/30 05:35:13]

We will do this by the next milestone.
We did not have the exact requirement and behavior specs about the dynamic
refresh for this feature.
Filed crbug.com/642247 .

+        'per_profile': True,
+      },
+      'example_value': True
+    },
+    {
       'name': 'AudioOutputAllowed',
       'type': 'main',
       'schema': { 'type': 'boolean' },