Add Startup.BrowserView.FirstPaint / .CompositingEnded histograms.

chrome/browser/metrics/browser_window_histogram_helper.h

+// 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 CHROME_BROWSER_METRICS_BROWSER_WINDOW_HISTOGRAM_HELPER_H_
+#define CHROME_BROWSER_METRICS_BROWSER_WINDOW_HISTOGRAM_HELPER_H_
+
+#include <memory>
+
+#include "base/macros.h"
+#include "base/scoped_observer.h"
+#include "base/time/time.h"
+#include "ui/compositor/compositor_observer.h"
+
+// Class that encapsulates logic of recording
+// Startup.BrowserWindow.FirstPaint* histograms.

[Alexei Svitkine (slow), 2017/04/19 16:52:16]

Add a sentence here to mention that this is not in
components/startup_metrics_utils because of a dependency on ui/compositor.

[themblsha, 2017/04/19 17:27:40]

Done.

+class BrowserWindowHistogramHelper : public ui::CompositorObserver {
+ public:
+  ~BrowserWindowHistogramHelper() override;
+
+  // Call this when the Browser finishes painting its UI, and the user will see
+  // it after next Compositor frame swap.
+  // |compositor| is the compositor that composites the just-painted Browser
+  // widget, or nullptr in Cocoa (we're using CoreAnimation's compositor there).
+  // Returned object should stay alive until next |OnCompositingStarted|
+  // callback.
+  static std::unique_ptr<BrowserWindowHistogramHelper>
+  RecordValueAndMaybeCreateInstanceOnBrowserPainted(ui::Compositor* compositor);

[Alexei Svitkine (slow), 2017/04/19 16:52:16]

Nit: The "Record value" part is also under Maybe since you don't do it unless
it's the first time - so put the Maybe at the front.

MaybeRecordValueAndCreateInstanceOnBrowserPaint()

(No need to use Painted - just Paint - since the name is long enough as it is
and Paint is the name of the vent.)

[themblsha, 2017/04/19 17:27:40]

Good point, done.

+
+ private:
+  explicit BrowserWindowHistogramHelper(ui::Compositor* compositor);
+
+  // ui::CompositorObserver:
+

[Alexei Svitkine (slow), 2017/04/19 16:52:16]

Nit: Remove empty line

[themblsha, 2017/04/19 17:27:41]

Done.

+  void OnCompositingDidCommit(ui::Compositor* compositor) override {}
+  void OnCompositingStarted(ui::Compositor* compositor,
+                            base::TimeTicks start_time) override;
+  void OnCompositingLockStateChanged(ui::Compositor* compositor) override {}
+  void OnCompositingShuttingDown(ui::Compositor* compositor) override;
+
+  ScopedObserver<ui::Compositor, ui::CompositorObserver> scoped_observer_;
+
+  DISALLOW_COPY_AND_ASSIGN(BrowserWindowHistogramHelper);
+};
+
+#endif  // CHROME_BROWSER_METRICS_BROWSER_WINDOW_HISTOGRAM_HELPER_H_