update_engine: Make utils::FormatTimeDelta() work with negative values.

BUG=chromium:401862
TEST=New unit test + unit tests pass.

Change-Id: I89b7ec72c11b792d4353eb6b36b23eac4f82107d
Reviewed-on: https://chromium-review.googlesource.com/212878
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/utils_unittest.cc b/utils_unittest.cc
index 53e4428..f23dd5c 100644
--- a/utils_unittest.cc
+++ b/utils_unittest.cc
@@ -459,6 +459,8 @@
   EXPECT_EQ(utils::FormatTimeDelta(base::TimeDelta::FromSeconds(200000) +
                                    base::TimeDelta::FromMilliseconds(1)),
             "2d7h33m20.001s");
+  EXPECT_EQ(utils::FormatTimeDelta(base::TimeDelta::FromSeconds(-1)),
+            "-1s");
 }
 
 TEST(UtilsTest, TimeFromStructTimespecTest) {