Make installed_loader UMA less wasteful.

extensions/common/manifest.h

Index: extensions/common/manifest.h
diff --git a/extensions/common/manifest.h b/extensions/common/manifest.h
index a50e724c5792814e0c4c99962da0e031d1d04638..77f5197f3c71af304468944d450fc9e870e285c7 100644
--- a/extensions/common/manifest.h
+++ b/extensions/common/manifest.h
@@ -47,6 +47,7 @@ class Manifest {
                         // internal implementation detail of chrome, but
                         // installed from an update URL like the *DOWNLOAD ones.
 
+    // New enum values must go above here.
     NUM_LOCATIONS
   };
 
@@ -62,7 +63,10 @@ class Manifest {
     // backwards compatibility, we can't remove support for packaged apps
     TYPE_LEGACY_PACKAGED_APP,
     TYPE_PLATFORM_APP,
-    TYPE_SHARED_MODULE
+    TYPE_SHARED_MODULE,
+
+    // New enum values must go above here.
+    NUM_LOAD_TYPES
   };
 
   // Given two install sources, return the one which should take priority