Re-enable performance_browser_tests on win8
Re-enable performance_browser_tests on win8

The latest comment in bug 475940, though dated, seems to indicate the
tests run just fine.  The only way to know for sure is to re-enable them
and confirm they are no longer flaky.

BUG=475940
TBR=simonhatch@chromium.org

Review URL: https://codereview.chromium.org/1553573002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#367112}

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