Fix number of buckets in Tabs.ScrubDistance UMA histogram.

Fix the number of buckets in Tabs.ScrubDistance. Currently the value
19 falls in the bucket with value 18 which is confusing. Results can be
checked on uma.googleplex.com.

BUG=

Review URL: https://codereview.chromium.org/551893002

Cr-Commit-Position: refs/heads/master@{#296957}

chrome/browser/ui/views/ash/tab_scrubber.cc

@@ -274,7 +274,7 @@ void TabScrubber::FinishScrub(bool activate) {
       int distance =
           std::abs(
               highlighted_tab_ - browser_->tab_strip_model()->active_index());
-      UMA_HISTOGRAM_CUSTOM_COUNTS("Tabs.ScrubDistance", distance, 0, 20, 20);
+      UMA_HISTOGRAM_CUSTOM_COUNTS("Tabs.ScrubDistance", distance, 0, 20, 21);
       browser_->tab_strip_model()->ActivateTabAt(highlighted_tab_, true);
     }
     tab_strip->RemoveObserver(this);