[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.

(cherry picked from commit e705a095439d0658fb81803396d4dcdb4c368c64)

Bug: 973088, 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-Original-Commit-Position: refs/heads/master@{#667714}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653649
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/branch-heads/3809@{#226}
Cr-Branched-From: d82dec1a818f378c464ba307ddd9c92133eac355-refs/heads/master@{#665002}

• chrome/test/chromedriver/commands.cc[diff]
• chrome/test/chromedriver/commands_unittest.cc[diff]
• chrome/test/chromedriver/server/http_handler.cc[diff]
• chrome/test/chromedriver/session_commands.cc[diff]
• chrome/test/chromedriver/session_commands.h[diff]
• chrome/test/chromedriver/session_thread_map.h[diff]