Cleanup: Pull some browser keep alive functions into its own file.

chrome/browser/lifetime/browser_keep_alive.h

+// Copyright 2015 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_LIFETIME_BROWSER_KEEP_ALIVE_H_
+#define CHROME_BROWSER_LIFETIME_BROWSER_KEEP_ALIVE_H_
+
+namespace browser_lifetime {
+
+// Tells the BrowserList to keep the application alive after the last Browser
+// closes. This is implemented as a count, so callers should pair their calls
+// to IncrementKeepAliveCount() with matching calls to DecrementKeepAliveCount()
+// when they no
+// longer need to keep the application running.
+void IncrementKeepAliveCount();

[sky, 2015/10/02 15:44:27]

I like factoring this code out into it's file. But I think code like this is
error prone. Is it possible to make IncrementKeepAliveCount/Decrement private
and instead force use through a class whose constructor IncrementKeepAliveCount
and destructor Decrements?

+
+// Stops keeping the application alive after the last Browser is closed.
+// Should match a previous call to IncrementKeepAliveCount().
+void DecrementKeepAliveCount();
+
+// Returns true if application will continue running after the last Browser
+// closes.
+bool WillKeepAlive();
+
+// Disable browser shutdown for unit tests.
+void DisableShutdownForTesting(bool disable_shutdown_for_testing);
+
+}  // namespace browser_lifetime
+
+#endif  // CHROME_BROWSER_LIFETIME_BROWSER_KEEP_ALIVE_H_
+