[build] Remove ability to disable TurboFan at build time.

This removes support for disabling TurboFan by default via the build
time {v8_disable_turbo} flag. We no longer need to be able to build
binaries with TurboFan disabled.

R=machenbach@chromium.org
BUG=v8:6408

Change-Id: I4062914c2bf823ab42250595ad67d1dc8da3f1d3
Reviewed-on: https://chromium-review.googlesource.com/528138Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45847}

BUILD.gn

@@ -24,9 +24,6 @@ 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 = ""
 
@@ -230,9 +227,6 @@ 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

@@ -448,13 +448,7 @@ DEFINE_BOOL(omit_map_checks_for_leaf_maps, true,
             "deoptimize the optimized code if the layout of the maps changes.")
 
 // Flags for TurboFan.
-#ifdef V8_DISABLE_TURBO
-// Allow to disable turbofan with a build flag after it's turned on by default.
-#define TURBO_BOOL false
-#else
-#define TURBO_BOOL true
-#endif
-DEFINE_BOOL(turbo, TURBO_BOOL, "enable TurboFan compiler")
+DEFINE_BOOL(turbo, true, "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,