[Extensions Cleanup] Remove ExecuteCodeFunction::resource_ member

It's only used in a single method. Just make it a function-local.

Bug: None
Change-Id: I0ebcc1f9a6ff8d724490e2e41aa7c4fe35096f86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518196


Auto-Submit: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824809}