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..c805934522ec1bef09a77b99eb575debe7bf61ea
--- /dev/null
+++ b/cc/input/scroller_size_metrics.h
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+// 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;
+static constexpr int kScrollerSizeBucketCount = 50;
+
+}  // namespace cc
+
+#endif  // CC_INPUT_SCROLLER_SIZE_METRICS_H_