[heap] Turn on --write-protect-code-memory.

Bug: chromium:774108
Change-Id: I8001013d1ee75d0847fd6a804c9e37774d900992
Reviewed-on: https://chromium-review.googlesource.com/957088
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51873}

src/flag-definitions.h

@@ -672,7 +672,7 @@ DEFINE_BOOL(incremental_marking_wrappers, true,
 DEFINE_BOOL(trace_unmapper, false, "Trace the unmapping")
 DEFINE_BOOL(parallel_scavenge, true, "parallel scavenge")
 DEFINE_BOOL(trace_parallel_scavenge, false, "trace parallel scavenge")
-DEFINE_BOOL(write_protect_code_memory, false, "write protect code memory")
+DEFINE_BOOL(write_protect_code_memory, true, "write protect code memory")
 #ifdef V8_CONCURRENT_MARKING
 #define V8_CONCURRENT_MARKING_BOOL true
 #else