[build] Introduce build flag to disable turbofan.

BUG=chromium:682617

Change-Id: I58800e5cf1926fabc684e0f140c2856175877bda
Reviewed-on: https://chromium-review.googlesource.com/451418Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43652}

BUILD.gn

@@ -23,6 +23,9 @@ declare_args() {
   # Sets -DV8_ENABLE_FUTURE.
   v8_enable_future = false
 
+  # Sets -DV8_DISABLE_TURBO.
+  v8_disable_turbo = false
+
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = ""
 
@@ -203,6 +206,9 @@ config("features") {
   if (v8_enable_future) {
     defines += [ "V8_ENABLE_FUTURE" ]
   }
+  if (v8_disable_turbo) {
+    defines += [ "V8_DISABLE_TURBO" ]
+  }
   if (v8_enable_gdbjit) {
     defines += [ "ENABLE_GDB_JIT_INTERFACE" ]
   }

src/flag-definitions.h

@@ -441,7 +441,14 @@ DEFINE_BOOL(omit_map_checks_for_leaf_maps, true,
             "deoptimize the optimized code if the layout of the maps changes.")
 
 // Flags for TurboFan.
-DEFINE_BOOL(turbo, false, "enable TurboFan compiler")
+#ifdef V8_DISABLE_TURBO
+// Allow to disable turbofan with a build flag after it's turned on by default.
+#define TURBO_BOOL false
+#else
+// TODO(mvstanton): Turn on turbofan here.
+#define TURBO_BOOL false
+#endif
+DEFINE_BOOL(turbo, TURBO_BOOL, "enable TurboFan compiler")
 DEFINE_BOOL(turbo_sp_frame_access, false,
             "use stack pointer-relative access to frame wherever possible")
 DEFINE_BOOL(turbo_preprocess_ranges, true,