This directs all asm.js traffic via the Wasm backend.
Make asm.js console output less noisy.

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

Review-Url: https://codereview.chromium.org/2624813002
Cr-Original-Original-Commit-Position: refs/heads/master@{#42194}
Committed: https://chromium.googlesource.com/v8/v8/+/946cc371ed4b34d1a9f5cc615b14c41b652562ad
Review-Url: https://codereview.chromium.org/2624813002
Cr-Original-Commit-Position: refs/heads/master@{#42244}
Committed: https://chromium.googlesource.com/v8/v8/+/3169fb94c98953f002908974a606b51a35178046
Review-Url: https://codereview.chromium.org/2624813002
Cr-Commit-Position: refs/heads/master@{#42300}