headless: Replace is_headless gn setting with an args template

Replace the is_headless setting with a GN args template file which
explicitly lists the settings needed to build the headless
configuration. To make this work, some of the existing settings need to
be moved into declare_args to make them overridable.

BUG=546953

Review URL: https://codereview.chromium.org/1845473003

Cr-Commit-Position: refs/heads/master@{#384549}
7 files changed