Add unit test for the Settings API Bubble.

extensions/browser/extension_prefs.cc

Index: extensions/browser/extension_prefs.cc
diff --git a/extensions/browser/extension_prefs.cc b/extensions/browser/extension_prefs.cc
index 24f7e5831ba809aba5fac2d5fb2aea243ecfef5e..53d800ca0d56ba17f495ffad418172959d52efba 100644
--- a/extensions/browser/extension_prefs.cc
+++ b/extensions/browser/extension_prefs.cc
@@ -1897,10 +1897,8 @@ void ExtensionPrefs::SetInstallSignature(
 std::string ExtensionPrefs::GetInstallParam(
     const std::string& extension_id) const {
   const base::DictionaryValue* extension = GetExtensionPref(extension_id);
-  if (!extension) {
-    NOTREACHED();
+  if (!extension)  // Expected during unit testing.
     return std::string();
-  }
   std::string install_parameter;
   if (!extension->GetString(kPrefInstallParam, &install_parameter))
     return std::string();