update_engine: Replace g_usleep() with usleep().

We use g_usleep() to wait 100ms delay in unittests. This patch uses
directly usleep() for that purpose since the delay is less than one
second.

BUG=chromium:499886
TEST=Unittets still pass.

Change-Id: I7fee3d6709791dd80432dbecf6f119df22b0ec7d
Reviewed-on: https://chromium-review.googlesource.com/291459
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/subprocess_unittest.cc b/subprocess_unittest.cc
index eb95133..2660456 100644
--- a/subprocess_unittest.cc
+++ b/subprocess_unittest.cc
@@ -193,7 +193,7 @@
                 << strerror(errno);
       break;
     }
-    g_usleep(kSleepTime.InMicroseconds());
+    usleep(kSleepTime.InMicroseconds());
     total_wait_time += kSleepTime;
   }
   close(temp_fd);