Stage write-protected code behind the --future flag.

R=hablich@chromium.org
BUG=v8:6792,chromium:774108

Change-Id: I6bb376ea5d1c72f668398fb75f8b2bbea5fdff20
Reviewed-on: https://chromium-review.googlesource.com/771551Reviewed-by: Michael Hablich <hablich@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49404}

src/flag-definitions.h

@@ -270,6 +270,7 @@ DEFINE_BOOL(future, FUTURE_BOOL,
             "not-too-far future")
 
 DEFINE_IMPLICATION(future, preparser_scope_analysis)
+DEFINE_IMPLICATION(future, write_protect_code_memory)
 
 // Flags for experimental implementation features.
 DEFINE_BOOL(allocation_site_pretenuring, true,