Add a policy to disable mounting of external storage.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 455b2e16daff1c75e83bf7645d9ad0ee140a485c..5542e4c987b2665c81bbd7dbcda8f9123f1dbc0c 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -15257,6 +15257,9 @@ Battery full
       <message name="IDS_MULTIPART_DEVICE_UNSUPPORTED_DEFAULT_MESSAGE" desc="Text of notification message which is shown when user inserts removable device (SD card, USB key...)">
        Sorry, at least one partition on your external storage device could not be mounted.
       </message>
+      <message name="IDS_EXTERNAL_STORAGE_DISABLED_MESSAGE" desc="Text of notification message which is shown when user inserts removable device but external storage is disabled by policy">
+       Sorry, your administrator has disabled external storage on your account.
+      </message>
 
       <!-- Formatting device notifications -->
       <message name="IDS_FORMATTING_OF_DEVICE_PENDING_TITLE" desc="Text of notification message which is shown when formatting process of some device is pending">