Set default number of sweeper threads to two.

BUG=

Review URL: https://codereview.chromium.org/12301020

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13749 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/flag-definitions.h

@@ -440,7 +440,7 @@ DEFINE_bool(track_gc_object_stats, false,
             "track object counts and memory usage")
 DEFINE_bool(parallel_sweeping, true, "enable parallel sweeping")
 DEFINE_bool(concurrent_sweeping, false, "enable concurrent sweeping")
-DEFINE_int(sweeper_threads, 1,
+DEFINE_int(sweeper_threads, 2,
            "number of parallel and concurrent sweeping threads")
 DEFINE_bool(parallel_marking, false, "enable parallel marking")
 DEFINE_int(marking_threads, 1, "number of parallel marking threads")