Add DownloadDB suffix for LevelDB.Open.*

DownloadDB is based on levelDB, and we need to understand the initialization
failures

BUG=873735

Change-Id: I0408732645cdefbb94a9ce43df3f8053a5a5a024
Reviewed-on: https://chromium-review.googlesource.com/1173070
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582989}

tools/metrics/histograms/histograms.xml

@@ -120942,6 +120942,7 @@ uploading your change for review.
   <suffix name="BudgetManager"
       label="Database for storing budget information for origins."/>
   <suffix name="DomDistillerStore" label="Databases for DomDistillerStore"/>
+  <suffix name="DownloadDB" label="Databases for in-progress download."/>
   <suffix name="FeatureEngagementTrackerAvailabilityStore"
       label="Database for FeatureEngagementTracker feature availability."/>
   <suffix name="FeatureEngagementTrackerEventStore"