Record size of scroller that user scrolls

cc/input/scroller_size.h

Index: cc/input/scroller_size.h
diff --git a/cc/input/scroller_size.h b/cc/input/scroller_size.h
new file mode 100644
index 0000000000000000000000000000000000000000..f1fcca99249acd4127f14ed6d8347307f019c0e2
--- /dev/null
+++ b/cc/input/scroller_size.h
@@ -0,0 +1,16 @@
+// 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_H_

[pdr., 2017/04/19 21:03:50]

Nit: scroller_size_metrics.h?

[yigu, 2017/04/20 18:14:22]

Done.

+#define CC_INPUT_SCROLLER_SIZE_H_
+
+namespace cc {
+
+// Use the two constants to add scroller size related UMA metrics.
+static constexpr int kMaxScrollerSize = 200000;  // Approximately 400px * 500px

[pdr., 2017/04/19 21:03:50]

I don't understand what this constant is for. Why do we need a maximum scroller
size for the metrics?

[yigu, 2017/04/19 21:10:05]

We are about to use the metrics and other stuff to decide whether it's worth it
to composite small scrollers. If a scroller (e.g. 50 * 50) is hardly getting
scrolled, we may not need to composite it. For big scrollers (e.g. larger than
400 * 500), they don't matter that much to this experiment so we don't record
the exact size for those scrollers.

[pdr., 2017/04/19 21:13:47]

Can you add that as a comment above the constant?

[flackr, 2017/04/20 07:15:01]

Also max scroller size sounds a bit misleading. Maybe
kScrollerSizeLargestBucket. We will still record larger scrollers but they will
get capped into this bucket right?

[yigu, 2017/04/20 18:14:22]

Done.

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