[gn] Fix v8_enable_gdbjit default

Broke in https://codereview.chromium.org/2116913002/ due
to rebase.

BUG=chromium:474921
NOTRY=true
TBR=jochen@chromium.org, dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2156993002
Cr-Commit-Position: refs/heads/master@{#37841}

build_overrides/v8.gni

@@ -16,7 +16,7 @@ if (((v8_current_cpu == "x86" ||
       v8_current_cpu=="x87") &&
      (is_linux || is_mac)) ||
     (v8_current_cpu == "ppc64" && is_linux)) {
-  v8_enable_gdbjit = true
+  v8_enable_gdbjit_default = true
 }
 
 v8_imminent_deprecation_warnings_default = true