[ios] Fix warning when running "gn gen".

ios/BUILD.gn

Index: ios/BUILD.gn
diff --git a/ios/BUILD.gn b/ios/BUILD.gn
index 7ba2d563681e362e6889e8a675a9deb65b3e9ad2..29356b2fb0096f2a1643625af4ecc8386b15922c 100644
--- a/ios/BUILD.gn
+++ b/ios/BUILD.gn
@@ -13,6 +13,7 @@ declare_args() {
   # the overrides are never used.
   v8_extra_library_files = []
   v8_experimental_extra_library_files = []
+  v8_enable_inspector = true
   v8_enable_gdbjit = false
   v8_imminent_deprecation_warnings = false
 }
@@ -20,6 +21,7 @@ declare_args() {
 # Prevent warnings for unused build args above.
 assert(v8_extra_library_files != 0)
 assert(v8_experimental_extra_library_files != 0)
+assert(v8_enable_inspector)
 assert(!v8_enable_gdbjit)
 assert(!v8_imminent_deprecation_warnings)