Add startup metrics that measure the performance of the first web contents.

chrome/browser/metrics/first_web_contents_profiler.h

+// Copyright 2014 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_FIRST_WEB_CONTENTS_PROFILER_H_
+#define CHROME_BROWSER_METRICS_FIRST_WEB_CONTENTS_PROFILER_H_
+
+#include "base/memory/scoped_ptr.h"
+#include "content/public/browser/web_contents_observer.h"
+
+namespace content {
+class WebContents;
+}  // namespace content
+
+class FirstWebContentsProfilerDelegate {

[Alexei Svitkine (slow), 2014/12/03 20:05:57]

Nit: Any reason to not just nest it under FirstWebContentsProfiler so then it's
just FirstWebContentsProfiler::Delegate?

[erikchen, 2014/12/03 21:38:41]

sounds reasonable to me.

+ public:
+  // Called by the FirstWebContentsProfiler when it is finished collecting
+  // metrics. The delegate should take this opportunity to destroy the
+  // FirstWebContentsProfiler.
+  virtual void ProfilerFinishedCollectingMetrics() = 0;
+};
+
+// Measures start up performance of the first active web contents.
+class FirstWebContentsProfiler : public content::WebContentsObserver {
+ public:
+  // Creates a profiler for the active web contents. If there are multiple
+  // browsers, the first one is chosen. If there are no browsers, returns
+  // nullptr.
+  static scoped_ptr<FirstWebContentsProfiler> CreateProfilerForFirstWebContents(
+      FirstWebContentsProfilerDelegate* delegate);
+
+  // These metrics are currently experimental, and are only collected in the
+  // dev and canary channels.
+  static bool ShouldCollectMetrics();
+
+ private:
+  FirstWebContentsProfiler(content::WebContents* web_contents,
+                           FirstWebContentsProfilerDelegate* delegate);
+
+  // WebContentsObserver overrides.

[Alexei Svitkine (slow), 2014/12/03 20:05:57]

Nit: I think for new code, the preferred notation is:

// content::WebContentsObserver:

[erikchen, 2014/12/03 21:38:41]

sure, done. I kept the old style for
chrome/browser/metrics/chrome_browser_main_extra_parts_metrics.h, since the rest
of that file still uses the old style.

+  void DidFirstVisuallyNonEmptyPaint() override;
+  void DocumentOnLoadCompletedInMainFrame() override;
+  void WebContentsDestroyed() override;
+
+  // Whether this instance has finished collecting all of its metrics.
+  bool IsFinishedCollectingMetrics();
+
+  // Informs the delegate that this instance has finished collecting all of its
+  // metrics.
+  void FinishedCollectingMetrics();
+
+  // Whether the "NonEmptyPaint" metric has been collected.
+  bool collected_paint_metric_;
+
+  // Whether the "MainFrameLoad" metric has been collected.
+  bool collected_load_metric_;
+
+  // |delegate_| owns |this|.
+  FirstWebContentsProfilerDelegate* delegate_;
+
+  DISALLOW_COPY_AND_ASSIGN(FirstWebContentsProfiler);
+};
+
+#endif  // CHROME_BROWSER_METRICS_FIRST_WEB_CONTENTS_PROFILER_H_