update_engine: Fix fallthrough warning

Fix clang warnings which appears when -Wimplicit-fallthrough
flag gets added in the build.

BUG=chromium:904913
TEST=package build fixed with enabled warning

Cq-Depend: chromium:1643102
Change-Id: I5a90de65f383fd14e7793647912abeed735a029c
Reviewed-on: https://chromium-review.googlesource.com/1635712
Commit-Ready: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
diff --git a/update_attempter.cc b/update_attempter.cc
index 34b7bac..d97917a 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -25,6 +25,7 @@
 #include <vector>
 
 #include <base/bind.h>
+#include <base/compiler_specific.h>
 #include <base/files/file_util.h>
 #include <base/logging.h>
 #include <base/rand_util.h>
@@ -624,8 +625,10 @@
     case StagingCase::kNoSavedValue:
       prefs_->SetInt64(kPrefsWallClockStagingWaitPeriod,
                        staging_wait_time_.InDays());
+      FALLTHROUGH;
     case StagingCase::kSetStagingFromPref:
       omaha_request_params_->set_waiting_period(staging_wait_time_);
+      FALLTHROUGH;
     case StagingCase::kNoAction:
       // Staging is on, enable wallclock based wait so that its values get used.
       omaha_request_params_->set_wall_clock_based_wait_enabled(true);