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.

BUG=chromium:724678
CQ-DEPEND=CL:480928
Test: Build.

Change-Id: I01b70da87521d0884ed21acbd7ed3e0ff1e94357
Merged-In: I4dbaea4a2a19031375a8bf2415645a4f226dab57
diff --git a/common/test_utils.cc b/common/test_utils.cc
index e410283..85f78f9 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -256,7 +256,7 @@
                                      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(