Reenable SIMD.js support after the M40 rebase.

Commit 8c5d886 ("DEPS.xwalk: Roll v8-crosswalk (3bfc597 -> 11bb7b4)")
correctly rolled v8-crosswalk, but forgot to uncomment out the piece of
code that enables SIMD.js support by default.

BUG=XWALK-3210