Improve the UI for disabling off-store extension install.

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 6b77baceb528cc743ee187c628777d6d39277a74..0dead8e7f25b331c831271f17b3034a1d05aaf31 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -4411,6 +4411,9 @@ Update checks have repeatedly failed for the extension "<ph name="EXTENSION_NAME
       <message name="IDS_EXTENSION_INSTALL_UNEXPECTED_ID" desc="Error displayed during installation of a side-loaded app, extension, or theme when the ID of the referenced extension does not match the ID the developer declared during registration.">
         Expected ID "<ph name="EXPECTED_ID">$1<ex>bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb</ex></ph>", but ID was "<ph name="NEW_ID">$2<ex>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</ex></ph>".
       </message>
+      <message name="IDS_EXTENSION_INSTALL_DISALLOWED_ON_SITE" desc="Error displayed during installation of an extension when installation is not allowed from the current site.">
+        Extensions cannot be installed from this web site.
+      </message>
       <message name="IDS_EXTENSION_INSTALL_UNEXPECTED_VERSION" desc="Error displayed during installation of a side-loaded app, extension, or theme when the version of the referenced extension does not match the version the developer declared during registration.">
         Expected version "<ph name="EXPECTED_VERSION">$1<ex>2.0</ex></ph>", but version was "<ph name="NEW_ID">$2<ex>1.0</ex></ph>".
       </message>