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Re-organize (ChromeDriver) http server into its own class so that
http handler can be a friend of http server. Then we can handle
the http server methods by posting on cmd thread to call
http handler methods. The reason to post on cmd thread is
that we can ensure thread safety when manipulating session map.

Also, pass cmd task runner and pointer to session map to
InitSessionParams so that session_commands can manipulate
the session map upon ExecuteInitSession.

Will make follow up CL to add condition check before manipulating the
session map. Follow up CL will also need to parse user WebSocketUrl
parameter.

There is also pending discussion about the behavior WebSocket connection
if the session terminates in the CL code review comments.

Bug: chromedriver:3588
Change-Id: Ibbd8b12842cf2e5b93d67bf68e1b869bb93f20c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382299
Commit-Queue: Shengfa Lin <shengfa@google.com>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803644}