Add UMA logging of extension creation flags.

extensions/common/extension.h

Index: extensions/common/extension.h
diff --git a/extensions/common/extension.h b/extensions/common/extension.h
index 3354fa625c2a170096a871cc86cb70c285e6b5fd..8a7e78e59129220e3a0d9d8c453b6917820a1d91 100644
--- a/extensions/common/extension.h
+++ b/extensions/common/extension.h
@@ -117,6 +117,8 @@ class Extension : public base::RefCountedThreadSafe<Extension> {
     virtual ~ManifestData() {}
   };
 
+  // Do not change the order of entries or remove entries in this list
+  // as this is used in UMA_HISTOGRAM_ENUMERATIONs about extensions.
   enum InitFromValueFlags {
     NO_FLAGS = 0,
 
@@ -169,8 +171,13 @@ class Extension : public base::RefCountedThreadSafe<Extension> {
     // be placed in a special OEM folder in the App Launcher. Note: OEM apps are
     // also installed by Default (i.e. WAS_INSTALLED_BY_DEFAULT is also true).
     WAS_INSTALLED_BY_OEM = 1 << 10,
+
+    // When adding new flags, make sure to update kInitFromValueFlagBits.
   };
 
+  // This is the highest bit index of the flags defined above.
+  static const int kInitFromValueFlagBits;
+
   static scoped_refptr<Extension> Create(const base::FilePath& path,
                                          Manifest::Location location,
                                          const base::DictionaryValue& value,