Properly define v8_enable_inspector and its override in GN.

gni/v8.gni

 # Copyright 2016 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/v8_target_cpu.gni")
+import("//build_overrides/v8.gni")
 
 declare_args() {
   # Indicate if valgrind was fetched as a custom deps to make it available on
   # swarming.
   v8_has_valgrind = false
 
   # Indicate if gcmole was fetched as a hook to make it available on swarming.
   v8_gcmole = false
 
   # Turns on compiler optimizations in V8 in Debug build.
   v8_optimized_debug = true
 
   # Support for backtrace_symbols on linux.
   v8_enable_backtrace = ""
 
   # Enable the snapshot feature, for fast context creation.
   # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
   v8_use_snapshot = true
 
   # Use external files for startup data blobs:
   # the JS builtins sources and the start snapshot.
   v8_use_external_startup_data = ""
 
   # Enable ECMAScript Internationalization API. Enabling this feature will
   # add a dependency on the ICU library.
   v8_enable_i18n_support = true
+
+  # Enable inspector. See include/v8-inspector.h.
+  v8_enable_inspector = v8_enable_inspector_override
 }
 
 if (v8_use_external_startup_data == "") {
   # If not specified as a gn arg, use external startup data by default if
   # a snapshot is used and if we're not on ios.
   v8_use_external_startup_data = v8_use_snapshot && !is_ios
 }
 
 if (v8_enable_backtrace == "") {
   v8_enable_backtrace = is_debug && !v8_optimized_debug
@@ skipping 70 matching lines @@
 }
 
 template("v8_component") {
   component(target_name) {
     forward_variables_from(invoker, "*", [ "configs" ])
     configs += invoker.configs
     configs -= v8_remove_configs
     configs += v8_add_configs
   }
 }