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

chrome/browser/lifetime/application_lifetime_aura.cc

Index: chrome/browser/lifetime/application_lifetime_aura.cc
diff --git a/chrome/browser/lifetime/application_lifetime_aura.cc b/chrome/browser/lifetime/application_lifetime_aura.cc
index 75d67bae8588d0be4b9bddcc62e7089cd362d412..7ca523dddfc045707195cfa96005e682a829f7b2 100644
--- a/chrome/browser/lifetime/application_lifetime_aura.cc
+++ b/chrome/browser/lifetime/application_lifetime_aura.cc
@@ -7,6 +7,7 @@
 #include "ash/shell.h"
 #include "base/command_line.h"
 #include "chrome/browser/browser_process.h"
+#include "chrome/browser/lifetime/browser_keep_alive.h"
 #include "chrome/browser/notifications/notification_ui_manager.h"
 #include "chrome/common/chrome_switches.h"
 #include "ui/aura/client/capture_client.h"
@@ -35,7 +36,7 @@ void HandleAppExitingForPlatform() {
   if (!base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kDisableZeroBrowsersOpenForTests)) {
     // App is exiting, call DecrementKeepAliveCount() on behalf of Aura Shell.
-    DecrementKeepAliveCount();
+    browser_lifetime::DecrementKeepAliveCount();
     // Make sure we have notified the session manager that we are exiting.
     // This might be called from FastShutdown() or CloseAllBrowsers(), but not
     // if something prevents a browser from closing before SetTryingToQuit()