Update description of BrowsingDataCounter's |pref_|

Remove part of the description of BrowsingDataCounter's |pref_| that's no longer true and irrelevant.

Bug: none
Change-Id: Ibfb47522b33b5ca92f9e29191c79b7070d425e38
Reviewed-on: https://chromium-review.googlesource.com/c/1358459Reviewed-by: Christian Dullweber <dullweber@chromium.org>
Commit-Queue: Huanzhong Huang <huanzhong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614324}

components/browsing_data/core/counters/browsing_data_counter.h

@@ -169,7 +169,6 @@ class BrowsingDataCounter {
   Callback callback_;
 
   // The boolean preference indicating whether this data type is to be deleted.
-  // If false, we will not count it.
   BooleanPrefMember pref_;
 
   // The integer preference describing the time period for which this data type