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: refs/heads/master@{#377928}
1 file changed