ChromeOS: fix crash in HashWallpaperFilesIdStr.

chromeos/cryptohome/system_salt_getter.h

Index: chromeos/cryptohome/system_salt_getter.h
diff --git a/chromeos/cryptohome/system_salt_getter.h b/chromeos/cryptohome/system_salt_getter.h
index a42dd251f8bf96067ae360e6c7f9e91ec7b84823..a77d85c1e601a9256b2ac186ede05cbc9cba579c 100644
--- a/chromeos/cryptohome/system_salt_getter.h
+++ b/chromeos/cryptohome/system_salt_getter.h
@@ -39,6 +39,10 @@ class CHROMEOS_EXPORT SystemSaltGetter {
   // an empty string (e.g. errors in D-Bus layer)
   void GetSystemSalt(const GetSystemSaltCallback& callback);
 
+  // Adds another callback to be called when system salt is received.
+  // (If system salt is available, closure will be called immediately).
+  void AddOnSystemSaltReady(const base::Closure& closure);
+
   // Returns pointer to binary system salt if it is already known.
   // Returns nullptr if system salt is not known.
   const RawSalt* GetRawSalt() const;
@@ -61,6 +65,9 @@ class CHROMEOS_EXPORT SystemSaltGetter {
   RawSalt raw_salt_;
   std::string system_salt_;
 
+  // List of callbacks waiting for system salt ready event.
+  std::vector<base::Closure> on_system_salt_ready_;
+
   base::WeakPtrFactory<SystemSaltGetter> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SystemSaltGetter);