Revert "Enable gn check in //headless."

This reverts commit 4133bde.

Reason for revert: broke headless_shell build.
To reproduce:
mkdir ./out/headless
cat >./out/headless
import("//build/args/headless.gn")
use_goma = true
is_component_build = false
is_debug = false

gn gen ./out/headless
autoninja -C ./out/headless headless_shell

Original change's description:
> Enable `gn check` in //headless.
>
> The main change in this CL is to move source files from
> :headless_non_renderer and :headless_renderer into
> :headless_shared_sources until none of the downstream targets have to
> depend on both :headless_non_renderer *and* :headless_renderer in order
> to resolve their includes; and continue moving source files into
> :headless_shared_sources as needed, until :headless_shared_sources does
> not have a circular dependency with either :headless_renderer or
> :headless_non_renderer.
>
> We also make :headless_renderer and
> :headless_non_renderer depend on :headless_shared_sources publicly, so
> that other :headless_* targets do not have to depend on one of those
> targets *and* :headless_shared_sources (which would cause a linker
> failure due to duplicate symbols). This has the side effect of exposing
> :headless_shared_sources symbols in //chrome:chrome_dll via
> :headless_renderer.
>
> Bug: 1158989, 1227143, 1227378
> Change-Id: I917ea3fddbb232abc27849fc3812ada7351e556b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3011653
> Reviewed-by: Joey Arhar <jarhar@chromium.org>
> Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Reviewed-by: Erik Staab <estaab@chromium.org>
> Auto-Submit: Chris Fredrickson <cfredric@chromium.org>
> Commit-Queue: Erik Staab <estaab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#904347}

Bug: 1158989, 1227143, 1227378
Change-Id: I88690af334467fa694008ab1291c9cd7b44af387
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3045885
Auto-Submit: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#904433}

.gn

@@ -65,6 +65,7 @@ default_args = {
 no_check_targets = [
   "//extensions/browser:*",  # 20 errors
   "//extensions:*",  # 28 errors
+  "//headless:*",  # 107 errors
   "//ppapi/proxy:ipc_sources",  # 13 errors
   "//ppapi/thunk:*",  # 1071 errors
   "//remoting/host/security_key:*",  # 10 errors