system-web-apps: fix negative install duration

The original CL that introduced the install duration metric contains
one typo, which cause the reported value to be the additive inverse of
the actual time.

Because UMA clamps this value to be within 1 millisecond and 3 minutes,
we always see 1 second in the UMA dashboard (and contradicting metrics
that some installation times out).

Fixed: 1083894
Change-Id: I7529bfb90fc737016d7c9948a7b8b6325ecb24a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207375
Auto-Submit: Jiewei Qian  <qjw@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#769909}

chrome/browser/web_applications/system_web_app_manager.cc

@@ -543,6 +543,9 @@ const std::string& SystemWebAppManager::CurrentLocale() const {
 void SystemWebAppManager::RecordSystemWebAppInstallMetrics(
     const std::map<GURL, InstallResultCode>& install_results,
     const base::TimeDelta& install_duration) const {
+  // Install duration should be positive.
+  DCHECK_GT(install_duration.InMicroseconds(), 0);
+
   // Record the time spent to install system web apps.
   if (!shutting_down_) {
     base::UmaHistogramMediumTimes(kInstallDurationHistogramName,
@@ -605,7 +608,7 @@ void SystemWebAppManager::OnAppsSynchronized(
   }
 
   const base::TimeDelta install_duration =
-      install_start_time - base::TimeTicks::Now();
+      base::TimeTicks::Now() - install_start_time;
 
   if (IsEnabled()) {
     // TODO(qjw): Figure out where install_results come from, decide if