DEPS.xwalk: Bring in v8-crosswalk.

Start tracking v8-crosswalk in src/v8. v8-crosswalk is a V8 fork along the
lines of chromium-crosswalk and blink-crosswalk. For now its main purpose is
to have all the SIMD.js optimizations some folks have been working on. Other
than that, it should be the usual V8 we all know.
Merge pull request #1699 from rakuco/pull-v8-crosswalk

DEPS.xwalk: Bring in v8-crosswalk.