Add a data source field for volumes.

chrome/browser/chromeos/file_system_provider/registry.h

Index: chrome/browser/chromeos/file_system_provider/registry.h
diff --git a/chrome/browser/chromeos/file_system_provider/registry.h b/chrome/browser/chromeos/file_system_provider/registry.h
index 0483e4594e089d6022073f31247bd2f4c06183ae..9072905cc9107507e346139a7c3928ba2d567ce0 100644
--- a/chrome/browser/chromeos/file_system_provider/registry.h
+++ b/chrome/browser/chromeos/file_system_provider/registry.h
@@ -24,6 +24,7 @@ namespace file_system_provider {
 extern const char kPrefKeyFileSystemId[];
 extern const char kPrefKeyDisplayName[];
 extern const char kPrefKeyWritable[];
+extern const char kPrefKeySource[];
 extern const char kPrefKeySupportsNotifyTag[];
 extern const char kPrefKeyWatchers[];
 extern const char kPrefKeyWatcherEntryPath[];
@@ -37,6 +38,11 @@ class ProvidedFileSystemInfo;
 // Registers preferences to remember registered file systems between reboots.
 void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);
 
+// Converts a file system's data source value back and forth between Source and
+// std::string types.
+std::string SourceToString(Source source);
+bool StringToSource(const std::string& source, Source* result);
+
 // Remembers and restores file systems in a persistent storage.
 class Registry : public RegistryInterface {
  public: