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This fixes deps and enables gn check for:

  //extensions/shell:app_shell
  //extensions/shell:app_shell_lib
  //extensions/shell:app_shell_unittests
  //extensions/shell/browser/system_logs

which lets us specify //extensions/shell/* in //.gn.

Some headers (esp. for NaCl) need to be annotated with // nogncheck
because gn doesn't run the preprocessor:
https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#what-gets-checked

Bug: 367595
Change-Id: If37c5935cb993aa13f7586ee8ff17cdb23dddfc3
Reviewed-on: https://chromium-review.googlesource.com/958282Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543768}