GN: Fix nacl_helper and nacl_helper_nonsfi path
GN: Fix nacl_helper and nacl_helper_nonsfi path

https://codereview.chromium.org/1530663002 broke the build of
nacl_helper and nacl_helper_nonsfi by moving NaCl targets to
subdirectories without updating all GN files.

R=brettw@chromium.org
BUG=462791

Review URL: https://codereview.chromium.org/1530763006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365658}

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