Properly define v8_enable_inspector and its override in GN.

build_overrides/v8.gni

Index: build_overrides/v8.gni
diff --git a/build_overrides/v8.gni b/build_overrides/v8.gni
index df8320d5d1746df85a411f5a9e76359c2bec0058..919b519b71b58e755a44833f245e0f46faded305 100644
--- a/build_overrides/v8.gni
+++ b/build_overrides/v8.gni
@@ -2,14 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/config/features.gni")
-import("//build/config/ui.gni")
 import("//build/config/v8_target_cpu.gni")
-import("//gni/v8.gni")
-
-if (is_android) {
-  import("//build/config/android/config.gni")
-}
 
 if (((v8_current_cpu == "x86" || v8_current_cpu == "x64" ||
       v8_current_cpu == "x87") && (is_linux || is_mac)) ||
@@ -24,9 +17,4 @@ v8_extra_library_files = [ "//test/cctest/test-extra.js" ]
 v8_experimental_extra_library_files =
     [ "//test/cctest/test-experimental-extra.js" ]
 
-declare_args() {
-  # Enable inspector. See include/v8-inspector.h.
-  v8_enable_inspector = true
-}
-
-v8_enable_inspector_override = v8_enable_inspector
+v8_enable_inspector_override = true