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/prefs_unittest.cc b/common/prefs_unittest.cc
index d94623a..1000131 100644
--- a/common/prefs_unittest.cc
+++ b/common/prefs_unittest.cc
@@ -21,6 +21,7 @@
 #include <string>
 
 #include <base/files/file_util.h>
+#include <base/files/scoped_temp_dir.h>
 #include <base/macros.h>
 #include <base/strings/string_util.h>
 #include <base/strings/stringprintf.h>
@@ -41,19 +42,17 @@
 class PrefsTest : public ::testing::Test {
  protected:
   void SetUp() override {
-    ASSERT_TRUE(base::CreateNewTempDirectory("auprefs", &prefs_dir_));
+    ASSERT_TRUE(temp_dir_.CreateUniqueTempDir());
+    prefs_dir_ = temp_dir_.path();
     ASSERT_TRUE(prefs_.Init(prefs_dir_));
   }
 
-  void TearDown() override {
-    base::DeleteFile(prefs_dir_, true);  // recursive
-  }
-
   bool SetValue(const string& key, const string& value) {
     return base::WriteFile(prefs_dir_.Append(key), value.data(),
                            value.length()) == static_cast<int>(value.length());
   }
 
+  base::ScopedTempDir temp_dir_;
   base::FilePath prefs_dir_;
   Prefs prefs_;
 };