[Chromedriver] Make new session payload spec compliant

Bug: chromedriver:1942
Change-Id: Ifba46ddae6f006e2d92fd7c6806a4781bcd2857c
Reviewed-on: https://chromium-review.googlesource.com/621208
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Jonathon Kereliuk <kereliuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495825}
2 files changed