Turn scavenge_reclaim_unmodified_objects on by default

Embedders that rely on unmodified wrappers to survive should pass the command-line flag --noscavenge_reclaim_unmodified_objects

BUG=4880
LOG=yes

Review URL: https://codereview.chromium.org/1839243005

Cr-Commit-Position: refs/heads/master@{#35164}

src/flag-definitions.h

@@ -708,7 +708,7 @@ DEFINE_BOOL(verify_heap, false, "verify heap pointers before and after GC")
 #endif
 DEFINE_BOOL(move_object_start, true, "enable moving of object starts")
 DEFINE_BOOL(memory_reducer, true, "use memory reducer")
-DEFINE_BOOL(scavenge_reclaim_unmodified_objects, false,
+DEFINE_BOOL(scavenge_reclaim_unmodified_objects, true,
             "remove unmodified and unreferenced objects")
 DEFINE_INT(heap_growing_percent, 0,
            "specifies heap growing factor as (1 + heap_growing_percent/100)")