[heap] Preserve optimized code maps accross GCs.

This disables the eager flushing of optimized code maps during full GCs
and instead treats entries in those tables weakly by default. Note that
for now entries are removed when one of its components dies.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1430293002

Cr-Commit-Position: refs/heads/master@{#32045}

src/flag-definitions.h

@@ -380,7 +380,7 @@ DEFINE_INT(stress_runs, 0, "number of stress runs")
 DEFINE_BOOL(lookup_sample_by_shared, true,
             "when picking a function to optimize, watch for shared function "
             "info, not JSFunction itself")
-DEFINE_BOOL(flush_optimized_code_cache, true,
+DEFINE_BOOL(flush_optimized_code_cache, false,
             "flushes the cache of optimized code for closures on every GC")
 DEFINE_BOOL(inline_construct, true, "inline constructor calls")
 DEFINE_BOOL(inline_arguments, true, "inline functions with arguments object")