Add a histogram to measure browser window display

chrome/common/startup_metric_utils.h

Index: chrome/common/startup_metric_utils.h
diff --git a/chrome/common/startup_metric_utils.h b/chrome/common/startup_metric_utils.h
new file mode 100644
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+++ b/chrome/common/startup_metric_utils.h
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+// Copyright (c) 2012 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_COMMON_STARTUP_METRIC_UTILS_H_
+#define CHROME_COMMON_STARTUP_METRIC_UTILS_H_
+
+// Utility functions to support metric collection for browser startup.
+
+namespace startup_metric_utils {
+
+// Returns true if any UI other than the browser window has been displayed
+// so far.  Useful to test if UI has been displayed before the first browser
+// window was shown, which would invalidate any surrounding timing metrics.
+bool WasNonBrowserUIDisplayed();
+
+// Call this when displaying UI that might potentially delay the appearance
+// of the initial browser window on Chrome startup.
+//
+// Note on usage: This function is idempotent and its overhead is low enough
+// in comparison with UI display that it's OK to call it on every
+// UI invocation regardless of whether the browser window has already
+// been displayed or not.
+void SetNonBrowserUIDisplayed();
+
+}  // namespace startup_metric_utils
+
+#endif  // CHROME_COMMON_STARTUP_METRIC_UTILS_H_