Add GoogleDocsOfflineExtension to UKMs dashboard.

ukm.xml is updated to add GoogleDocsOfflineExtension metrics to UKMs
dashboard.

Bug: 588766
Change-Id: I46f09d9ab3e68304610a0f4567b0c3329258f4ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094984Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ramin Halavati <rhalavati@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748897}

tools/metrics/ukm/ukm.xml

@@ -3750,6 +3750,13 @@ be describing additional metrics about the same event.
     <summary>
       Marks a resource request from a website.
     </summary>
+    <aggregation>
+      <history>
+        <statistics>
+          <enumeration/>
+        </statistics>
+      </history>
+    </aggregation>
   </metric>
 </event>