GN: Add assert that ash/BUILD.gn referenced only on supported platforms
GN: Add assert that ash/BUILD.gn referenced only on supported platforms

This helps to ensure that targets not supported on a platform are not included in the "all" target.

BUG=None

Review URL: https://codereview.chromium.org/1542633006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367472}

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