Add support for single sample metrics.

content/common/metrics.mojom

Index: content/common/metrics.mojom
diff --git a/content/common/metrics.mojom b/content/common/metrics.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..61fad8806424f714d556255d8639c8a3c06c41d7
--- /dev/null
+++ b/content/common/metrics.mojom
@@ -0,0 +1,10 @@
+// Copyright 2016 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.
+
+module content.mojom;
+
+interface OneShotMetricHost {
+  Initialize(string name, int32 min, int32 max, uint32 bucket_count);
+  SetSample(int32 sample);
+};