Turn on job-based optimizing compiler

BUG=v8:3608
R=yangguo@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/1001313002

Cr-Commit-Position: refs/heads/master@{#27175}

src/flag-definitions.h

@@ -367,7 +367,7 @@ DEFINE_BOOL(optimize_for_in, true, "optimize functions containing for-in loops")
 
 DEFINE_BOOL(concurrent_recompilation, true,
             "optimizing hot functions asynchronously on a separate thread")
-DEFINE_BOOL(job_based_recompilation, false,
+DEFINE_BOOL(job_based_recompilation, true,
             "post tasks to v8::Platform instead of using a thread for "
             "concurrent recompilation")
 DEFINE_BOOL(trace_concurrent_recompilation, false,