[OnionSoup] Create mojom traits for typemapping network.mojom.URLRequest

This CL makes network.mojom.URLRequest not a [Native] mojom struct:
  - define all its fields well.
  - create the mojom struct traits for keeping its typemap to
    network::ResourceRequest.
  - the legacy IPC struct traits of network::ResourceRequest becomes
    useless, remove it.

This makes network.mojom.URLRequest no more than a mirror of
network::ResourceRequest, by defining each field as almost the same type
with the mirror filed in network::ResourceRequest, this is not the final
definition but a start point for refining, which may happen in the
future when we create the typemap to blink::ResourceRequest for Blink
variant.

The future plan would be:
 - make network.mojom.URLRequestBody not a [Native] one, keeping its
   typemap to network::ResourceRequestBody.
 - for Blink variant create a typemap to blink::EncodedFormData for
   network.mojom.URLRequestBody.
 - for Blink variant create a typemap to blink::ResourceRequest for
   network.mojom.URLRequest.

BUG=860403

Change-Id: I4b962cd42d093d3f03070a2eeb197dabfcbd9d01
Reviewed-on: https://chromium-review.googlesource.com/c/1381735
Commit-Queue: Leon Han <leon.han@intel.com>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622375}
11 files changed