Revert of gn BUILD fixes for disabling enable_extensions and use_ash feature flags. (patchset #7 id:120001 of https://codereview.chromium.org/1950003002/ )

Reason for revert:
Breaks the chromium.chromiumos gn builder.

Repro:
gn args out/Default

Set to:
is_component_build = true
use_goma = true
target_os = "chromeos"

gn gen reports error:
ERROR Unresolved dependencies.
//:both_gn_and_gyp(//build/toolchain/cros:clang_target)
  needs //extensions/shell:app_shell(//build/toolchain/cros:clang_target)

See https://build.chromium.org/p/chromium.chromiumos/buildstatus?builder=Linux%20ChromiumOS%20GN%20%28dbg%29&number=22111

Original issue's description:
> gn BUILD fixes for disabling enable_extensions and use_ash feature flags.
>
> BUG=
>
> Committed: https://crrev.com/2e75c702de464a5cc1cd2272ef720fbe1068c141
> Cr-Commit-Position: refs/heads/master@{#399693}

TBR=dgozman@chromium.org,dpranke@google.com,dpranke@chromium.org,mfoltz@chromium.org,ben@chromium.org,asaka@opera.com
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review-Url: https://codereview.chromium.org/2064173002
Cr-Commit-Position: refs/heads/master@{#399721}
5 files changed