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/test_utils.cc b/common/test_utils.cc
index fb22c80..3447cdc 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -248,7 +248,7 @@
                                      string* mnt_path,
                                      unsigned long flags) {  // NOLINT - long
   EXPECT_TRUE(temp_dir_.CreateUniqueTempDir());
-  *mnt_path = temp_dir_.path().value();
+  *mnt_path = temp_dir_.GetPath().value();
 
   string loop_dev;
   loop_binder_.reset(