[gn] Set correct defaults for some gn args

build_overrides/v8.gni

Index: build_overrides/v8.gni
diff --git a/build_overrides/v8.gni b/build_overrides/v8.gni
index 5dcd67257a8e40443db653b45cea8e2394efdbe3..7ba4d58384975606ab439462df617497c1cfa0fa 100644
--- a/build_overrides/v8.gni
+++ b/build_overrides/v8.gni
@@ -16,10 +16,10 @@ if (((v8_target_cpu == "x86" ||
       v8_target_cpu=="x87") &&
      (is_linux || is_mac)) ||
     (v8_target_cpu == "ppc64" && is_linux)) {
-  v8_enable_gdbjit = true
+  v8_enable_gdbjit_default = true
 }
 
-v8_imminent_deprecation_warnings = true
+v8_imminent_deprecation_warnings_default = true
 
 # Add simple extras solely for the purpose of the cctests.
 v8_extra_library_files = [ "//test/cctest/test-extra.js" ]