Add Startup.BrowserView.FirstPaint / .CompositingEnded histograms.

chrome/browser/ui/views/frame/browser_view_histogram_helper.cc

+// 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.
+
+#include "chrome/browser/ui/views/frame/browser_view_histogram_helper.h"
+
+#include "components/startup_metric_utils/browser/startup_metric_utils.h"
+#include "ui/compositor/compositor.h"
+
+BrowserViewHistogramHelper::BrowserViewHistogramHelper() {}
+
+BrowserViewHistogramHelper::~BrowserViewHistogramHelper() {
+  RemoveCompositorObserver();
+}
+
+void BrowserViewHistogramHelper::OnDidPaintChildren(
+    ui::Compositor* compositor) {
+  if (!did_first_paint_) {
+    did_first_paint_ = true;
+
+    startup_metric_utils::RecordBrowserViewFirstPaint(base::TimeTicks::Now());
+
+    compositor->AddObserver(this);

[Peter Kasting, 2017/03/24 22:39:17]

Nit: Prefer a ScopedObserver to manual add/remove calls.

This will also allow removing the code from the destructor, and removing
|compositor_|.  (The existing DCHECKs that use it don't really buy much, I
wouldn't bother trying to recreate them with the scoped observer.)

[themblsha, 2017/03/31 14:32:31]

Done.

+    compositor_ = compositor;
+  }
+}
+
+void BrowserViewHistogramHelper::RemoveCompositorObserver() {
+  if (compositor_) {
+    compositor_->RemoveObserver(this);
+    compositor_ = nullptr;
+  }
+}
+
+void BrowserViewHistogramHelper::OnCompositingEnded(
+    ui::Compositor* compositor) {
+  DCHECK_EQ(compositor_, compositor);
+
+  startup_metric_utils::RecordBrowserViewFirstPaintCompositingEnded(
+      base::TimeTicks::Now());
+
+  RemoveCompositorObserver();
+}
+
+void BrowserViewHistogramHelper::OnCompositingShuttingDown(
+    ui::Compositor* compositor) {
+  DCHECK_EQ(compositor_, compositor);
+  RemoveCompositorObserver();
+}