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diff --git a/docshell/base/timeline/AutoTimelineMarker.h b/docshell/base/timeline/AutoTimelineMarker.h
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+++ b/docshell/base/timeline/AutoTimelineMarker.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_AutoTimelineMarker_h_
+#define mozilla_AutoTimelineMarker_h_
+
+#include "mozilla/GuardObjects.h"
+#include "mozilla/RefPtr.h"
+
+class nsIDocShell;
+
+namespace mozilla {
+
+// # AutoTimelineMarker
+//
+// An RAII class to trace some task in the platform by adding a start and end
+// timeline marker pair. These markers are then rendered in the devtools'
+// performance tool's waterfall graph.
+//
+// Example usage:
+//
+//     {
+//       AutoTimelineMarker marker(mDocShell, "Parse CSS");
+//       nsresult rv = ParseTheCSSFile(mFile);
+//       ...
+//     }
+class MOZ_RAII AutoTimelineMarker
+{
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER;
+
+  // The name of the marker we are adding.
+  const char* mName;
+
+  // The docshell that is associated with this marker.
+  RefPtr<nsIDocShell> mDocShell;
+
+public:
+  AutoTimelineMarker(nsIDocShell* aDocShell,
+                     const char* aName MOZ_GUARD_OBJECT_NOTIFIER_PARAM);
+  ~AutoTimelineMarker();
+
+  AutoTimelineMarker(const AutoTimelineMarker& aOther) = delete;
+  void operator=(const AutoTimelineMarker& aOther) = delete;
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_AutoTimelineMarker_h_ */