Fix crash on OnCustomLinksChanged DCHECK

OnCustomLinksChanged can be called before the list of custom links is
initialized in CustomLinksManager, i.e. if the kCustomLinksList pref is
synced before kCustomLinksInitialized.

Bug: 922333
Change-Id: Ie723565a65b4d599fdc55bca76d53f8ec60668fa
Reviewed-on: https://chromium-review.googlesource.com/c/1416111
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623774}
1 file changed