Turn on job-based sweeping

Embedders that don't use a v8::Platform yet will have to do so.

BUG=v8:3104
LOG=y
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/443043002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23562 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/flag-definitions.h

@@ -532,7 +532,7 @@ DEFINE_BOOL(parallel_sweeping, false, "enable parallel sweeping")
 DEFINE_BOOL(concurrent_sweeping, true, "enable concurrent sweeping")
 DEFINE_INT(sweeper_threads, 0,
            "number of parallel and concurrent sweeping threads")
-DEFINE_BOOL(job_based_sweeping, false, "enable job based sweeping")
+DEFINE_BOOL(job_based_sweeping, true, "enable job based sweeping")
 #ifdef VERIFY_HEAP
 DEFINE_BOOL(verify_heap, false, "verify heap pointers before and after GC")
 #endif