This will be used for disassembling individual wasm function for
showing them in devtools.
The PrintAst function now also optionally provides an offset table
mapping from byte offset to line and column in the generated text.

R=titzer@chromium.org, ahaas@chromium.org
BUG=chromium:613110

Review-Url: https://codereview.chromium.org/2050213002
Cr-Commit-Position: refs/heads/master@{#37026}