CFI: download LLVM Gold plugin, if requested by env var or GYP_DEFINES.
CFI: download LLVM Gold plugin, if requested by env var or GYP_DEFINES.

BUG=chromium:536159

Review URL: https://codereview.chromium.org/1385403006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353437}

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