Fix clang/win build after https://codereview.chromium.org/1543173002/
Fix clang/win build after https://codereview.chromium.org/1543173002/

That change made metrics_aura.cc include build_config.h.  Before,
nothing did, so `defined(OS_WIN)` was always false (even on Windows!).
Now that it isn't, clang-cl complains that a constant is unused
on Windows, so move it to a more local scope.

BUG=82385
R=tfarina@chromium.org
TBR=avi

Review URL: https://codereview.chromium.org/1544083002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366771}

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