[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/712598Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48458}

src/flag-definitions.h

@@ -577,7 +577,7 @@ 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, 2,
+DEFINE_INT(retain_maps_for_n_gc, 0,
            "keeps maps alive for <n> old space garbage collections")
 DEFINE_BOOL(trace_gc, false,
             "print one trace line following each garbage collection")