Currently, V8 uses the same size counter to measure function byte 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=bbudge@google.com,bradnelson@chromium.org

Review-Url: https://codereview.chromium.org/2780783003
Cr-Commit-Position: refs/heads/master@{#44205}