[cleanup] Fix uses deprecated v8 Object::Get in gin

Bug: v8:7283, v8:8562
Change-Id: I037bfb3818fdb9f92d5d9e0a8a71152ccdddade3
Reviewed-on: https://chromium-review.googlesource.com/c/1450255
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628380}
2 files changed