When the download folder does not exist, change the download folder to a user's "Downloads"

chrome/browser/download/download_prefs.h

Index: chrome/browser/download/download_prefs.h
diff --git a/chrome/browser/download/download_prefs.h b/chrome/browser/download/download_prefs.h
index cd7bad44e961bc81369d67d8d1d3d9be28c9dce3..578602d314d55a6b9397060e5fa368120be447c6 100644
--- a/chrome/browser/download/download_prefs.h
+++ b/chrome/browser/download/download_prefs.h
@@ -11,6 +11,10 @@
 #include "base/file_path.h"
 #include "chrome/browser/prefs/pref_member.h"
 
+namespace download_test_util {
+class ScopedDefaultDownloadDirectory;
+}
+
 class PrefService;
 
 // Stores all download-related preferences.
@@ -48,14 +52,26 @@ class DownloadPrefs {
 
   void ResetAutoOpen();
 
+  FilePath GetDefaultDownloadDirectory();
+
  private:
+  friend class download_test_util::ScopedDefaultDownloadDirectory;
+
   void SaveAutoOpenState();
 
+  // This method can be used only through ScopedDefaultDownloadDirectory.
+  void OverrideDefaultDownloadDirectory(
+      const FilePath& override_default_download_dir);
+
+  // This method can be used only through ScopedDefaultDownloadDirectory.
+  void UnOverrideDefaultDownloadDirectory();
+
   PrefService* prefs_;
 
   BooleanPrefMember prompt_for_download_;
   FilePathPrefMember download_path_;
   IntegerPrefMember save_file_type_;
+  FilePath override_default_download_dir_;
 
   // Set of file extensions to open at download completion.
   struct AutoOpenCompareFunctor {