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This was a workaround for VS's before 2013 that lack va_copy, and is
no longer required. The unittest is removed as it seems unnecessary
to test the standard library's implementation of va_copy.

R=thakis@chromium.org
TBR=cpu@chromium.org

Review URL: https://codereview.chromium.org/802533002

Cr-Commit-Position: refs/heads/master@{#307991}