zlib: Remove Windows XP workaround for CPU feature detection
zlib: Remove Windows XP workaround for CPU feature detection

XP didn't have InitOnceExecuteOnce() so it was necessary to implement an
equivalent using atomic compare-and-swap. With XP support no longer necessary
the CPU feature detection code can now use InitOnceExecuteOnce() directly.

TEST=net_unittests(GZipUnitTest)
BUG=587964

Review URL: https://codereview.chromium.org/1711683003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#377928}

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