We should not install the SIMD object on the global object unless
support for SIMD is actually turned on by the flag. Otherwise calling
SIMD.Float32x4() will hit unreachable code. We accidentally SIMD.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/1240223002

Cr-Commit-Position: refs/heads/master@{#29749}