DNR: Update Extensions.DeclarativeNetRequest.LoadRulesetResult histogram description.

r645865 added support for dynamic rules to the Declarative Net Request API. This
changed the conditions under which the histogram
Extensions.DeclarativeNetRequest.LoadRulesetResult is logged. Update the
histogram description to reflect the same. Using the same histogram should be
fine since the API is still in beta and has seen minimal usage.

BUG=930961

Change-Id: I3c8aaa82a459ccfc0d33553d8688f79a4540f286
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548556
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Auto-Submit: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#647084}

tools/metrics/histograms/histograms.xml

@@ -34661,7 +34661,8 @@ uploading your change for review.
   <owner>karandeepb@chromium.org</owner>
   <summary>
     Records the result of loading an extension ruleset for the Declarative Net
-    Request API. Emitted whenever an extension with a ruleset is loaded.
+    Request API. Emitted whenever an extension with a ruleset is loaded or the
+    dynamic rules for an extension are updated.
   </summary>
 </histogram>