Currently, V8 uses the same size counter to measure module sizes for both asm.js
and WASM. This CL separates the counter into two separate counters, and then
uses the appropriate counter when decoding.

BUG=chromium:704922
R=mtrofin@chromium.org,bradnelson@chromium.org

Review-Url: https://codereview.chromium.org/2777203005
Cr-Commit-Position: refs/heads/master@{#44211}