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/791033007/)

Reason for revert:
attempt reland after landing a fix

Original issue's description:
> Revert of Turn on job-based recompilation (patchset #2 id:20001 of https://codereview.chromium.org/832183004/)
>
> Reason for revert:
> revert
>
> Original issue's description:
> > Turn on job-based recompilation
> >
> > BUG=v8:3608
> > LOG=y
> > R=yangguo@chromium.org
> >
> > Committed: https://crrev.com/715552409b4a08504301cf1ad4cb2e45a815f01b
> > Cr-Commit-Position: refs/heads/master@{#25972}
>
> TBR=yangguo@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=v8:3608
>
> Committed: https://crrev.com/28a5a4865183769cbed9d42ad3fdc1b1d8bae3a3
> Cr-Commit-Position: refs/heads/master@{#25975}

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

Review URL: https://codereview.chromium.org/840023002

Cr-Commit-Position: refs/heads/master@{#25994}

src/flag-definitions.h

@@ -352,10 +352,9 @@ 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_IMPLICATION(job_based_recompilation, concurrent_recompilation)
 DEFINE_BOOL(trace_concurrent_recompilation, false,
             "track concurrent recompilation")
 DEFINE_INT(concurrent_recompilation_queue_length, 8,