gpu: Fix dependency cycle between service_sources and config_sources.

gpu: Fix dependency cycle between service_sources and config_sources.

ERROR Dependency cycle:
  //gpu/command_buffer/service:service_sources ->
  //gpu/config:config_sources ->
  //gpu/command_buffer/service:service_sources

The problem was that gpu/command_buffer/service already include from
gpu/config, but gpu/config were also including from
gpu/command_buffer/service for gpu_switches.h, creating the above
dependency cycle.

By moving the required switch down to gpu/config, we broke this
dependency cycle.

With that 'gn check' passes for gpu/config with the following command
lines:

$ gn gen out-gn/Debug
$ gn check out-gn/Debug //gpu/config/*

gpu/command_buffer/service needs more work to pass 'gn check' though.

BUG=367595,376000
TEST=see above
R=sievers@chromium.org

Review URL: https://codereview.chromium.org/1103403002

Cr-Commit-Position: refs/heads/master@{#327344}