Add debug data for incorrect access to preferences by extension.

BUG=454513,467239

Review URL: https://codereview.chromium.org/1070063002

Cr-Commit-Position: refs/heads/master@{#324601}
diff --git a/extensions/browser/extension_pref_value_map.cc b/extensions/browser/extension_pref_value_map.cc
index d21a969..dd549d7 100644
--- a/extensions/browser/extension_pref_value_map.cc
+++ b/extensions/browser/extension_pref_value_map.cc
@@ -71,7 +71,10 @@
     bool incognito) const {
   ExtensionEntryMap::const_iterator ext = entries_.find(extension_id);
   if (ext == entries_.end()) {
-    NOTREACHED();
+    NOTREACHED() << "Extension " << extension_id
+                 << " is not registered but accesses pref " << pref_key
+                 << " (incognito: " << incognito << ")."
+                 << " http://crbug.com/454513";
     return false;
   }