Remove stale __GLIBC__ define from "mesa" GN target
Remove stale __GLIBC__ define from "mesa" GN target

__GLIBC__ define was removed from the "mesa" GYP target in
https://codereview.chromium.org/951983002 This CL removes the define in GN as
well

BUG=507294

Review URL: https://codereview.chromium.org/1439973002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359903}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk