GitLab

- Use the SDK-provided config.gni in //build/config/fuchsia/config.gni.
- Remove the fuchsia_sdk_pkg.gni and gen_fidl_response_file.py.
- Use the SDK-provided runtime_library_group to get runtime libraries.
- Temporarily have //build/config/fuchsia/fidl_lbrary.gni redirect to
  the SDK-provided version.
- Temporarily update gen_build_defs.py to generate "redirect" rules,
  from the old Chromium targets to the SDK-provided ones.

Follow-up CLs will migrate away from gen_build_defs.py to direct
dependencies on the SDK-provided libraries, etc.

Bug: 1050703
Change-Id: Ib970aa99fe1a2808d30a536dd0d370db6ab8c4d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057307
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741942}