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After http://crrev.com/575378 webrunner used --type=web-context argument
to distinguish between web::ContextProvider and web::Context processes.
This approach didn't work properly because there is a lot of code that
assumes that process_type is not set (or empty) in the browser
process, which corresponds to web::Context. This CL removes
--type=web-context. Instead webrunner uses presense of the Context
request handle in the startup info to distinguish between the two
process types.

Also updated ContextProviderMain() to initialize MessageLoop as it's
required on all threads that use FIDL.

Bug: 852145
Change-Id: I2d308c712530f5802a4783cbebcffa49fade326d
Reviewed-on: https://chromium-review.googlesource.com/1141174
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576140}