Only allow extension pages to be displayed in the sidebar to be consistent...

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
===================================================================
--- chrome/app/generated_resources.grd	(revision 72219)
+++ chrome/app/generated_resources.grd	(working copy)
@@ -3621,6 +3621,9 @@
       <message name="IDS_EXTENSION_LOAD_OPTIONS_PAGE_FAILED" desc="">
         Could not load options page '<ph name="OPTIONS_PAGE">$1<ex>page.html</ex></ph>'.
       </message>
+      <message name="IDS_EXTENSION_LOAD_SIDEBAR_PAGE_FAILED" desc="">
+        Could not load sidebar page '<ph name="SIDEBAR_PAGE">$1<ex>page.html</ex></ph>'.
+      </message>
       <if expr="os == 'win32'">
         <message name="IDS_EXTENSION_UNPACK_FAILED" desc="On windows, it is possible to mount a disk without the root of that disk having a drive letter.  The sandbox does not support this. See crbug/49530 .">
           Can not unpack extension.  To safely unpack an extension, there must be a path to your profile directory that starts with a drive letter and does not contain a junction, mount point, or symlink.  No such path exists for your profile.