Record size of scroller that user scrolls

cc/input/scroller_size_metrics.h

Index: cc/input/scroller_size_metrics.h
diff --git a/cc/input/scroller_size_metrics.h b/cc/input/scroller_size_metrics.h
new file mode 100644
index 0000000000000000000000000000000000000000..6e59313a01c17ba64a50fa3fa64d3221852fb7c3
--- /dev/null
+++ b/cc/input/scroller_size_metrics.h
@@ -0,0 +1,21 @@
+// Copyright 2017 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.
+
+#ifndef CC_INPUT_SCROLLER_SIZE_METRICS_H_
+#define CC_INPUT_SCROLLER_SIZE_METRICS_H_
+
+namespace cc {
+
+// Use the two constants to add scroller size related UMA metrics.
+// The goal of the metrics is to find the frequency of scrollers of
+// different sizes that get scrolled. This experiment is aiming at small
+// scrollers therefore the big ones, e.g. larger than 400px * 500px, will get
+// capped into one bucket.
+static constexpr int kScrollerSizeLargestBucket =
+    200000;  // Approximately 400px * 500px, 250px * 800px, etc.

[flackr, 2017/04/21 00:56:35]

nit: You can remove this comment since the one above explains the size.

[yigu, 2017/04/21 12:20:33]

Done.

+static constexpr int kScrollerSizeBucketCount = 50;
+
+}  // namespace cc
+
+#endif  // CC_INPUT_SCROLLER_SIZE_METRICS_H_