Revert "Avoid crashes if all favicons already processed"

This reverts commit 98d99996.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Avoid crashes if all favicons already processed
> 
> We have evidence of DownloadCurrentCandidateOrAskFaviconService()
> crashing due to current_candidate() being null, at least until recent
> fixes.
> 
> Temporarily and as a precaution, we return early in case the problematic
> scenario is reproduced. We suspect this condition is not reached in
> trunk, but it's a simple workaround to cherry-pick.
> 
> TBR=pkotwicz@chromium.org
> 
> Bug: 828196
> Change-Id: I4cc78ba01780beb59ad01a7388062f6db93b864c
> Reviewed-on: https://chromium-review.googlesource.com/1000453
> Reviewed-by: Mikel Astiz <mastiz@chromium.org>
> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#549053}

TBR=mastiz@chromium.org,pkotwicz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 828196
Change-Id: I362142e484667d85c784a884c46a07494247e349
Reviewed-on: https://chromium-review.googlesource.com/1003152Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549260}

components/favicon/core/favicon_handler.cc

@@ -604,9 +604,6 @@ void FaviconHandler::OnFaviconDataForInitialURLFromFaviconService(
 }
 
 void FaviconHandler::DownloadCurrentCandidateOrAskFaviconService() {
-  if (!current_candidate())
-    return;
-
   DCHECK(image_download_request_.IsCancelled());
   DCHECK(manifest_download_request_.IsCancelled());
   DCHECK(current_candidate());