Only WASM has the notion of changing the maximum memory pages. This
CL corrects the UMA stats to only collect this data for WASM only.

BUG=v8:6321
R=bradnelson@chromium.org,bbudge@chromium.org

Review-Url: https://codereview.chromium.org/2845163002
Cr-Commit-Position: refs/heads/master@{#44988}