update_engine: Fix issues reported by clang-tidy

Fix uninitialized variables and some other minor issues reported by
clang-tidy.

BUG=chromium:982837
TEST=cros_workon_make update_engine --test

Change-Id: I305dedb058c9b0787ba2f68feff42afe6810a276
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2122683
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Andrew Lassalle <andrewlassalle@chromium.org>
Commit-Queue: Andrew Lassalle <andrewlassalle@chromium.org>
diff --git a/payload_consumer/delta_performer.cc b/payload_consumer/delta_performer.cc
index 3263ff7..11cf006 100644
--- a/payload_consumer/delta_performer.cc
+++ b/payload_consumer/delta_performer.cc
@@ -533,7 +533,7 @@
 #define OP_DURATION_HISTOGRAM(_op_name, _start_time)                         \
   LOCAL_HISTOGRAM_CUSTOM_TIMES(                                              \
       "UpdateEngine.DownloadAction.InstallOperation::" _op_name ".Duration", \
-      base::TimeTicks::Now() - _start_time,                                  \
+      (base::TimeTicks::Now() - _start_time),                                \
       base::TimeDelta::FromMilliseconds(10),                                 \
       base::TimeDelta::FromMinutes(5),                                       \
       20);