desktop-pwas: only update launcher if path exists

This change skips attempting PWA launcher update if the launcher path
does not exist.

Chrome updates PWA launchers by iterating over a list of enabled
bookmark apps. It determines each app's expected launcher path and
attempts to update it. This fails if the launcher path does not exist.

This change saves time spent attempting update when it's not
necessary, and should reduce noise in the WebApp.Launcher.UpdateResult
metric.

Bug: 1123064
Change-Id: Ibb30bb054ad338b84f8fe6ab66f919d800755ce4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382752Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#802849}

chrome/browser/web_applications/chrome_pwa_launcher/launcher_update.cc

@@ -144,7 +144,8 @@ void UpdatePwaLaunchers(std::vector<base::FilePath> launcher_paths) {
 
     // Make a hardlink or copy of |latest_version_path|, and replace the current
     // launcher with it.
-    ReplaceLauncherWithLatestVersion(path, latest_version_path, old_path);
+    if (base::PathExists(path))
+      ReplaceLauncherWithLatestVersion(path, latest_version_path, old_path);
   }
 }