Fix invalid native context reference in simd128-tag

The solution is to inline the native context load.

BUG=XWALK-2255
(cherry picked from commit 5aa07fb60139abfc847c4398c1b9c252711b2ce0)
Merge pull request #49 from huningxin/crosswalk-8/37.0.2062.76

Fix invalid native context reference in simd128-tag