mac: Unbreak building with the 10.11 SDK.
mac: Unbreak building with the 10.11 SDK.

enum and class definitions need to use MAC_OS_X_VERSION_MAX_ALLOWED, not
MAC_OS_X_VERSION_MIN_REQUIRED.  Move the enum and the definition up to the
"enum and typedef" definition and fix this.

BUG=548185
R=shrike@chromium.org

Review URL: https://codereview.chromium.org/1422843005 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#356380}

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