[Variations] Clarify a comment

Clarify that groups only have features automagically associated with them if
they don't directly specify *any* associated features. The comment was
previously worded for a single feature, and got only partially updated.

R=asvitkine@chromium.org

Bug: None
Change-Id: I6cd7e79fb78112c990a5f9ed6d4ca0d59531f159
Reviewed-on: https://chromium-review.googlesource.com/1005798
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549672}

components/variations/processed_study.cc

@@ -89,7 +89,7 @@ bool ValidateStudyAndComputeTotalProbability(
     return false;
   }
 
-  // Ensure that groups that don't explicitly enable/disable that feature get
+  // Ensure that groups that don't explicitly enable/disable any features get
   // associated with all features in the study (i.e. so "Default" group gets
   // reported).
   if (!features_to_associate.empty()) {