[build] Add build flag to activate the future configuration

BUG=chromium:682617

Change-Id: I96eb6bc982eca2d5f97585f3475583cac3fb338d
Reviewed-on: https://chromium-review.googlesource.com/448556Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43515}

BUILD.gn

@@ -20,6 +20,9 @@ declare_args() {
   # Print to stdout on Android.
   v8_android_log_stdout = false
 
+  # Sets -DV8_ENABLE_FUTURE.
+  v8_enable_future = false
+
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = ""
 
@@ -197,6 +200,9 @@ config("features") {
   if (v8_enable_disassembler) {
     defines += [ "ENABLE_DISASSEMBLER" ]
   }
+  if (v8_enable_future) {
+    defines += [ "V8_ENABLE_FUTURE" ]
+  }
   if (v8_enable_gdbjit) {
     defines += [ "ENABLE_GDB_JIT_INTERFACE" ]
   }

src/flag-definitions.h

@@ -254,7 +254,12 @@ HARMONY_STAGED(FLAG_STAGED_FEATURES)
 HARMONY_SHIPPING(FLAG_SHIPPING_FEATURES)
 #undef FLAG_SHIPPING_FEATURES
 
-DEFINE_BOOL(future, false,
+#ifdef V8_ENABLE_FUTURE
+#define FUTURE_BOOL true
+#else
+#define FUTURE_BOOL false
+#endif
+DEFINE_BOOL(future, FUTURE_BOOL,
             "Implies all staged features that we want to ship in the "
             "not-too-far future")
 DEFINE_IMPLICATION(future, turbo)