Revert of Revert of Turn on job based recompilation (patchset #1 id:1 of...

Revert of Revert of Turn on job based recompilation (patchset #1 id:1 of https://codereview.chromium.org/937263004/)

Reason for revert:
reland

Original issue's description:
> Revert of Turn on job based recompilation (patchset #1 id:1 of https://codereview.chromium.org/935243003/)
>
> Reason for revert:
> waterfall disagrees
>
> Original issue's description:
> > Turn on job based recompilation
> >
> > BUG=v8:3608
> > R=yangguo@chromium.org
> > LOG=y
> >
> > Committed: https://crrev.com/4ae82f64b09cdc0182cee3a5cc619a9117ac5fbc
> > Cr-Commit-Position: refs/heads/master@{#26826}
>
> TBR=yangguo@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=v8:3608
>
> Committed: https://crrev.com/35cc692aecdea54fb07479b8d7b71059851e07e8
> Cr-Commit-Position: refs/heads/master@{#26827}

TBR=yangguo@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:3608

Review URL: https://codereview.chromium.org/955723002

Cr-Commit-Position: refs/heads/master@{#26829}

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,