Remove utils::MakeTempDirectory().

In favor of base::ScopedTempDir, except for PostinstallRunnerAction,
where the temp directory needs to be removed for every partition.

ScopedDirRemover is also removed because it's no longer used.

Test: ./update_engine_unittests
Test: cros_workon_make update_engine --test
Bug: 26955860

Change-Id: I954e6e892aff0cf9f8434a77408dc3c9eb64c1b5
diff --git a/common/test_utils.cc b/common/test_utils.cc
index 13ce6f9..dfdc6b8 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -246,8 +246,8 @@
 ScopedLoopMounter::ScopedLoopMounter(const string& file_path,
                                      string* mnt_path,
                                      unsigned long flags) {  // NOLINT - long
-  EXPECT_TRUE(utils::MakeTempDirectory("mnt.XXXXXX", mnt_path));
-  dir_remover_.reset(new ScopedDirRemover(*mnt_path));
+  EXPECT_TRUE(temp_dir_.CreateUniqueTempDir());
+  *mnt_path = temp_dir_.path().value();
 
   string loop_dev;
   loop_binder_.reset(