Revise user-data-dir switch handling; avoid crashes.

chrome/browser/user_data_dir_extractor.h

Index: chrome/browser/user_data_dir_extractor.h
diff --git a/chrome/browser/user_data_dir_extractor.h b/chrome/browser/user_data_dir_extractor.h
index 5e8af0a12e63d53b44d511d12cec49186c3e0b54..3f4007708cb6b2cd2021d4ea04fac2b1e2fd69b6 100644
--- a/chrome/browser/user_data_dir_extractor.h
+++ b/chrome/browser/user_data_dir_extractor.h
@@ -5,6 +5,8 @@
 #ifndef CHROME_BROWSER_USER_DATA_DIR_EXTRACTOR_H_
 #define CHROME_BROWSER_USER_DATA_DIR_EXTRACTOR_H_
 
+#include "base/callback.h"
+
 namespace base {
 class FilePath;
 }
@@ -15,6 +17,12 @@ struct MainFunctionParams;
 
 namespace chrome {
 
+typedef base::Callback<base::FilePath()> GetUserDataDirCallback;
+
+// Tests may install a custom GetUserDataDir() callback to override behavior.
+void InstallCustomGetUserDataDirCallbackForTest(
+    GetUserDataDirCallback* callback);
+
 // Returns the user data dir. Must be called prior to InitializeLocalState().
 base::FilePath GetUserDataDir(const content::MainFunctionParams& parameters);