This is a flag useful for testing/perf investigations for asm-wasm.

We can dump the internal representation of a asm.js module (when asm-wasm is
enabled with --validate-asm) using --dump-wasm-module. We can't pick that
module afterwards because it contains non-compliant opcodes - used as
implementation detail of our asm-wasm pipeline.

This flag forces asm.js origin on the decoder.

BUG=

Review-Url: https://codereview.chromium.org/2656103003
Cr-Commit-Position: refs/heads/master@{#42716}