Revert of Turn off overapproximation of the weak closure again (patch…

…set nwjs#1 id:1 of https://codereview.chromium.org/1050443002/)

Reason for revert:
this indeed drops the max major gc time considerable, so turn it back on

Original issue's description:
> Turn off overapproximation of the weak closure again
>
> As long as we still have to process global handles, the impact is not
> yet worthwhile
>
> BUG=v8:3862
> R=hpayer@chromium.org
> LOG=y
>
> Committed: https://crrev.com/294cdc6aecbd7f76be68217da4b3d35901ebce4b
> Cr-Commit-Position: refs/heads/master@{#27570}

TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:3862

Review URL: https://codereview.chromium.org/1068723003

Cr-Commit-Position: refs/heads/master@{#27616}

src/flag-definitions.h

@@ -613,7 +613,7 @@ DEFINE_BOOL(age_code, true,
             "old code (required for code flushing)")
 DEFINE_BOOL(incremental_marking, true, "use incremental marking")
 DEFINE_BOOL(incremental_marking_steps, true, "do incremental marking steps")
-DEFINE_BOOL(overapproximate_weak_closure, false,
+DEFINE_BOOL(overapproximate_weak_closure, true,
             "overapproximate weak closer to reduce atomic pause time")
 DEFINE_INT(min_progress_during_object_groups_marking, 128,
            "keep overapproximating the weak closure as long as we discover at "