update_engine: Update libchrome APIS to r456626.

The new libchrome has been ported from Chromium and some APIs
have changed. Make necessary changes at call sites.

Change-Id: I8c3cea4510774c813a5d8fb10240918b5736884e
diff --git a/common/prefs_unittest.cc b/common/prefs_unittest.cc
index 73ceb00..aa2eb04 100644
--- a/common/prefs_unittest.cc
+++ b/common/prefs_unittest.cc
@@ -44,7 +44,7 @@
  protected:
   void SetUp() override {
     ASSERT_TRUE(temp_dir_.CreateUniqueTempDir());
-    prefs_dir_ = temp_dir_.path();
+    prefs_dir_ = temp_dir_.GetPath();
     ASSERT_TRUE(prefs_.Init(prefs_dir_));
   }