Merge src/leveldb changes for LevelDB 1.18.

src/leveldb/db/db_test.cc

@@ -626,7 +626,7 @@ TEST(DBTest, GetEncountersEmptyLevel) {
     //   * sstable B in level 2
     // Then do enough Get() calls to arrange for an automatic compaction
     // of sstable A.  A bug would cause the compaction to be marked as
-    // occuring at level 1 (instead of the correct level 0).
+    // occurring at level 1 (instead of the correct level 0).
 
     // Step 1: First place sstables in levels 0 and 2
     int compaction_count = 0;