sync: Define histogram ints for model types

sync/syncable/model_type_unittest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "sync/internal_api/public/base/model_type.h"
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 #include "base/test/values_test_util.h"
@@ skipping 68 matching lines @@
     int field_number = GetSpecificsFieldNumberFromModelType(iter.Get());
     EXPECT_EQ(iter.Get(),
               GetModelTypeFromSpecificsFieldNumber(field_number));
   }
 }
 
 TEST_F(ModelTypeTest, ModelTypeOfInvalidSpecificsFieldNumber) {
   EXPECT_EQ(UNSPECIFIED, GetModelTypeFromSpecificsFieldNumber(0));
 }
 
+TEST_F(ModelTypeTest, ModelTypeHistogramMapping) {
+  std::set<int> histogram_values;
+  ModelTypeSet all_types = ModelTypeSet::All();
+  for (ModelTypeSet::Iterator it = all_types.First(); it.Good(); it.Inc()) {
+    SCOPED_TRACE(ModelTypeToString(it.Get()));
+    int histogram_value = ModelTypeToHistogramInt(it.Get());
+
+    EXPECT_TRUE(histogram_values.insert(histogram_value).second)
+        << "Expected histogram values to be unique";
+
+    // This is not necessary for the mapping to be valid, but most instances of
+    // UMA_HISTOGRAM that use this mapping specify MODEL_TYPE_COUNT as the
+    // maximum possible value.  If you break this assumption, you should update
+    // those histograms.
+    EXPECT_LT(histogram_value, MODEL_TYPE_COUNT);
+  }
+}
+
 }  // namespace
 }  // namespace syncer