sync: Define histogram ints for model types

sync/internal_api/public/base/model_type.h

Index: sync/internal_api/public/base/model_type.h
diff --git a/sync/internal_api/public/base/model_type.h b/sync/internal_api/public/base/model_type.h
index a44ad198cfad440c9f05d4eed2d18c41883f9589..ebf76653f2241a471d676df8f362387fdb74b810 100644
--- a/sync/internal_api/public/base/model_type.h
+++ b/sync/internal_api/public/base/model_type.h
@@ -49,11 +49,6 @@ enum ModelType {
   // can be represented in the protocol using a specific Message type in the
   // EntitySpecifics protocol buffer.
   //
-  // WARNING: Modifying the order of these types or inserting a new type above
-  // these will affect numerous histograms that rely on the enum values being
-  // consistent. When adding a new type, add it to the end of the user model
-  // types section, but before the proxy types.
-  //
   // A bookmark folder or a bookmark URL object.
   BOOKMARKS,
   FIRST_USER_MODEL_TYPE = BOOKMARKS,  // Declared 2nd, for debugger prettiness.
@@ -244,6 +239,11 @@ FullModelTypeSet ToFullModelTypeSet(ModelTypeSet in);
 // the name of |model_type|.
 SYNC_EXPORT const char* ModelTypeToString(ModelType model_type);
 
+// Some histograms take an integer parameter that represents a model type.
+// The mapping from ModelType to integer is defined here.  It should match
+// the mapping from integer to labels defined in histograms.xml.
+SYNC_EXPORT int ModelTypeToHistogramInt(ModelType model_type);
+
 // Handles all model types, and not just real ones.
 //
 // Caller takes ownership of returned value.