[wasm][asm.js] Turn on asm.js -> WebAssembly by default.

BUG=v8:4203
R=danno@chromium.org,titzer@chromium.org,jochen@chromium.org

Review-Url: https://codereview.chromium.org/2625823006
Cr-Commit-Position: refs/heads/master@{#443289}
1 file changed