Do not CreateSentinel until after the process singleton has been grabbed by the current process.

chrome/browser/ui/startup/startup_browser_creator_browsertest.cc

Index: chrome/browser/ui/startup/startup_browser_creator_browsertest.cc
diff --git a/chrome/browser/ui/startup/startup_browser_creator_browsertest.cc b/chrome/browser/ui/startup/startup_browser_creator_browsertest.cc
index 13c10b0fa9b1337551ea34ad1a03740579fdea8f..1c6d2744839216f7f805a0850f402cbaf4844551 100644
--- a/chrome/browser/ui/startup/startup_browser_creator_browsertest.cc
+++ b/chrome/browser/ui/startup/startup_browser_creator_browsertest.cc
@@ -90,11 +90,6 @@ Browser* FindOneOtherBrowser(Browser* browser) {
 class StartupBrowserCreatorTest : public ExtensionBrowserTest {
  protected:
   virtual bool SetUpUserDataDirectory() OVERRIDE {
-    // Make sure the first run sentinel file exists before running these tests,
-    // since some of them customize the session startup pref whose value can
-    // be different than the default during the first run.
-    // TODO(bauerb): set the first run flag instead of creating a sentinel file.
-    first_run::CreateSentinel();
     return ExtensionBrowserTest::SetUpUserDataDirectory();
   }