Disable modifying extensions when in managed mode.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
===================================================================
--- chrome/app/generated_resources.grd	(revision 142678)
+++ chrome/app/generated_resources.grd	(working copy)
@@ -4695,6 +4695,9 @@
       <message name="IDS_EXTENSIONS_POLICY_CONTROLLED" desc="The text in the extensions UI informing the user that an extension is policy controlled">
         (This extension is managed and cannot be removed or disabled.)
       </message>
+      <message name="IDS_EXTENSIONS_LOCKED_MANAGED_MODE" desc="The error message (either shown in the extensions UI or logged) informing the user that extensions are controlled by managed mode">
+         Extensions cannot be modified in managed mode.
+      </message>
       <message name="IDS_GET_MORE_EXTENSIONS_DEPRECATED" desc="The link for getting more extensions. Displayed at bottom of extension management page when there is at least one extension installed.">
         <ph name="BEGIN_LINK">&lt;a target="blank_" href="$1"&gt;</ph>Get more extensions<ph name="END_LINK">&lt;/a&gt;<ex>&lt;/a&gt;</ex></ph>
       </message>