Various logical fixes for jumplist

This CL consists of the following three changes:

1) Move DeleteDirectoryContentAndLogResults() so that it runs only if
   jumplist_updater.BeginUpdate() succeeds, otherwise it's pointless.

2) Return early if any AddShellLink call fails. This is suspected to
   cause long execution time for jumplist_updater.

3) Decrease kFileDeleteLimit from 60 to 30. A user without jumplist
   issues should have at most 10 icon files in JumpListIcons folder as
   in crrev.com/2816113002. So changing kFileDeleteLimit to 30 won't
   affect "healthy" users. It'll give a better experience for users
   who have corrupted jumplist folders as the disk IO per jumplist
   update becomes less.

BUG=40407, 179576

Review-Url: https://codereview.chromium.org/2824103003
Cr-Commit-Position: refs/heads/master@{#465689}
3 files changed