[ChromeDriver] Allow POST with empty body in OSS mode

W3C WebDriver spec requires POST request to always have a non-empty
body. However, some legacy clients send empty POST body in some cases.
Updating ChromeDriver to handle such clients.

Bug: chromedriver:2943
Change-Id: If29dd990dbebc0b4fc7c5df35ac25e4a4bc47f30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648893
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667714}