Revert "[heap] Disable map retaining optimization."

This reverts commit 00ba1dca.

Reason for revert: perf regressions crbug.com/774903 

Original change's description:
> [heap] Disable map retaining optimization.
> 
> The optimization keeps dying maps alive for several GCs to mitigate
> code deoptimization with weak maps.
> 
> This patch disables the optimization to see if it still needed.
> 
> Bug: 
> Change-Id: Ie5717967ad56858e6ae546c90fde73e8d5bcc4ec
> Reviewed-on: https://chromium-review.googlesource.com/712598
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48458}

TBR=ulan@chromium.org,mlippautz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Id57d7239a041b03e02a35ccbf5830ff9838a6246
Reviewed-on: https://chromium-review.googlesource.com/733017Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48822}

src/flag-definitions.h

@@ -579,7 +579,7 @@ DEFINE_INT(max_old_space_size, 0, "max size of the old space (in Mbytes)")
 DEFINE_INT(initial_old_space_size, 0, "initial old space size (in Mbytes)")
 DEFINE_BOOL(gc_global, false, "always perform global GCs")
 DEFINE_INT(gc_interval, -1, "garbage collect after <n> allocations")
-DEFINE_INT(retain_maps_for_n_gc, 0,
+DEFINE_INT(retain_maps_for_n_gc, 2,
            "keeps maps alive for <n> old space garbage collections")
 DEFINE_BOOL(trace_gc, false,
             "print one trace line following each garbage collection")