GN: Add --depfile to generate_incremental_manifest.py action.
GN: Add --depfile to generate_incremental_manifest.py action.

Fixes the target always being considered dirty

BUG=

Review URL: https://codereview.chromium.org/1389663002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352399}

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