[Heap] Disable code flushing in preparation to remove it.

BUG=v8:6389,v8:6379,v8:6409

Change-Id: I24e0c8b6212f2cf2877d52f27eca0beb133afa1e
Reviewed-on: https://chromium-review.googlesource.com/508348Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45394}

src/flag-definitions.h

@@ -660,7 +660,7 @@ DEFINE_BOOL(trace_fragmentation_verbose, false,
 DEFINE_BOOL(trace_evacuation, false, "report evacuation statistics")
 DEFINE_BOOL(trace_mutator_utilization, false,
             "print mutator utilization, allocation speed, gc speed")
-DEFINE_BOOL(flush_code, true, "flush code that we expect not to use again")
+DEFINE_BOOL(flush_code, false, "flush code that we expect not to use again")
 DEFINE_BOOL(trace_code_flushing, false, "trace code flushing progress")
 DEFINE_BOOL(age_code, true,
             "track un-executed functions to age code and flush only "