Parse postinstall parameters from the payload metadata.

Payload v2 includes a description of the post-install command it should
run, while in payload v1 we use the default values. This patch mounts
the partition on the new top-level directory called /postinstall that
should already be created.

Bug: 27177071
TEST=FEATURES=test emerge-link update_engine

Change-Id: Iaedf3b01e5e1ad57c68bd316b4b6e79cbab35bb6
diff --git a/common/test_utils.cc b/common/test_utils.cc
index c09096b..a574863 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -260,7 +260,7 @@
   string loop_dev;
   loop_binder_.reset(new ScopedLoopbackDeviceBinder(file_path, &loop_dev));
 
-  EXPECT_TRUE(utils::MountFilesystem(loop_dev, *mnt_path, flags));
+  EXPECT_TRUE(utils::MountFilesystem(loop_dev, *mnt_path, flags, ""));
   unmounter_.reset(new ScopedFilesystemUnmounter(*mnt_path));
 }