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Reworks a lot of how arg parsing works so that there is less room for
failure (hopefully). Including:
 * Rename: --no-source-paths -> --no-output-directory
 * Make --ssargs-file top_args only.

Also allows passing --no-native via command-line and have that
work when .ssargs file is used.

Bug: 1132015
Change-Id: Ia4e46e056d9878e51d497041a231cb5dcde846fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429449
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810457}