by using C++11's default field initializers.

Bug: v8:8238
Change-Id: I3f5f4994114da61efb5b3c22681e6c472cf6e3dc
Reviewed-on: https://chromium-review.googlesource.com/c/1302054
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57074}