Properly define v8_enable_inspector and its override in GN.

gni/v8.gni

Index: gni/v8.gni
diff --git a/gni/v8.gni b/gni/v8.gni
index 3759572b935157e65237443c84ef456c508c2010..d2b4da825e630887619d6e84757b3f51b1fb60dd 100644
--- a/gni/v8.gni
+++ b/gni/v8.gni
@@ -4,6 +4,7 @@
 
 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
@@ -30,6 +31,9 @@ declare_args() {
   # 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 == "") {