DEPS.xwalk: Roll v8-crosswalk (ad45c4e -> 7fa32ec)

7fa32ec Fix invalid native context reference in simd128-tag

BUG=XWALK-2255
Merge pull request #2328 from huningxin/crosswalk-8

DEPS.xwalk: Roll v8-crosswalk (ad45c4e -> 7fa32ec)