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The bots still use 'cpu_arch' to set the architecture, and the build
file was keying off of build_cpu_arch to determine whether or not
to enable NaCl, so things were failing. Updating the the code to
'current_cpu' fixes the problem correctly.

TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/964003002

Cr-Commit-Position: refs/heads/master@{#318585}