Mention default and minimum for ExtensionCacheSize policy.

chrome/browser/chromeos/policy/proto/chrome_device_policy.proto

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package enterprise_management;
 
@@ skipping 592 matching lines @@
 message DeviceHeartbeatSettingsProto {
   // Whether the device should send heartbeat messages. The default is false.
   optional bool heartbeat_enabled = 1 [default = false];
 
   // How frequently devices send heartbeats back to server. The unit is in
   // milliseconds. The default is 2 minutes.
   optional int64 heartbeat_frequency = 2 [default = 120000];
 }
 
 message ExtensionCacheSizeProto {
-  // Specifies the maximum extension cache size, in bytes.
+  // Specifies the maximum extension cache size, in bytes. The default is 256
+  // MiB. The minimum allowed value is 1 MiB, smaller values will get ignored.
   optional int64 extension_cache_size = 1;
 }
 
 message LoginScreenDomainAutoCompleteProto {
   // If this policy is not configured or set to a blank string,
   // no autocomplete option during user sign-in flow will be shown.
   // If this policy is set to a string representing a domain name, an
   // autocomplete option during user sign-in will be shown allowing the user
   // to type in only his user name without the domain name extension. The user
   // will be able to overwrite this domain name extension.
@@ skipping 35 matching lines @@
   optional SystemUse24HourClockProto use_24hour_clock = 30;
   optional AutoCleanupSettigsProto auto_clean_up_settings = 31;
   optional SystemSettingsProto system_settings = 32;
   optional SAMLSettingsProto saml_settings = 33;
   optional RebootOnShutdownProto reboot_on_shutdown = 34;
   optional DeviceHeartbeatSettingsProto device_heartbeat_settings = 35;
   optional ExtensionCacheSizeProto extension_cache_size = 36;
   optional LoginScreenDomainAutoCompleteProto
       login_screen_domain_auto_complete = 37;
 }