Handle null fail_callback in SuggestionsService BlacklistURL.
Handle null fail_callback in SuggestionsService BlacklistURL.

e.g. base::Closure() is passed from InstantService.

BUG=524723

Review URL: https://codereview.chromium.org/1361753002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#350287}

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