Storage Service: Stop logging SessionStorageDatabase.Commit.

There is no need to log the obsolete SessionStorageDatabase.Commit
UMA metrics in a class only used by tests.

Bug: 975995
Change-Id: I7b6d23aaf91dcbc4fac22a7b48e7f7ce6f0ae5e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2169729
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763454}

components/services/storage/dom_storage/testing_legacy_session_storage_database.cc

@@ -219,9 +219,6 @@ bool TestingLegacySessionStorageDatabase::CommitAreaChanges(
 
   base::ScopedAllowBaseSyncPrimitivesForTesting allow_base_sync_primitives;
   leveldb::Status s = db_->Write(leveldb::WriteOptions(), &batch);
-  UMA_HISTOGRAM_ENUMERATION("SessionStorageDatabase.Commit",
-                            leveldb_env::GetLevelDBStatusUMAValue(s),
-                            leveldb_env::LEVELDB_STATUS_MAX);
   return DatabaseErrorCheck(s.ok());
 }