update_engine: fix BUILD.gn lint errors

Via:

  gn format BUILD.gn

BUG=none
TEST=presubmit

Change-Id: I64b028bfffe5c9aaac6086466d08602785d979e3
diff --git a/BUILD.gn b/BUILD.gn
index 224ad45..51a4ae0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -436,6 +436,7 @@
       "common/http_common.cc",
       "test_http_server.cc",
     ]
+
     # //common-mk:test should be on the top.
     # TODO(crbug.com/887845): Remove this after library odering issue is fixed.
     configs += [
@@ -449,6 +450,7 @@
     sources = [
       "test_subprocess.cc",
     ]
+
     # //common-mk:test should be on the top.
     # TODO(crbug.com/887845): Remove this after library odering issue is fixed.
     configs += [
@@ -542,6 +544,7 @@
       "update_manager/variable_unittest.cc",
       "update_manager/weekly_time_unittest.cc",
     ]
+
     # //common-mk:test should be on the top.
     # TODO(crbug.com/887845): Remove this after library odering issue is fixed.
     configs += [