Explicitly set v8_enable_gdbjit_default for v8 overrides.

This is a fix for https://codereview.chromium.org/2648383003 where
v8_enable_gdbjit should actually be v8_enable_gdbjit_default. This
was pointed out in the review for https://codereview.chromium.org/2654663003/

BUG=684096
R=machenbach@chromium.org

Review-Url: https://codereview.chromium.org/2652083002
Cr-Commit-Position: refs/heads/master@{#445835}

build_overrides/v8.gni

@@ -23,5 +23,5 @@ v8_experimental_extra_library_files =
     [ "//third_party/WebKit/Source/core/streams/WritableStream.js" ]
 
 v8_enable_inspector_override = true
-v8_enable_gdbjit = false
+v8_enable_gdbjit_default = false
 v8_imminent_deprecation_warnings_default = false