Update chromeos_build_instructions for GN and clean up.
Update chromeos_build_instructions for GN and clean up.

Adds a note for GYP_CHROMIUM_NO_ACTION=1 and includes a bit of cleanup.

BUG=none

Review URL: https://codereview.chromium.org/1902163002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388274}

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