Calculate memory size from the available heap. This avoids
the bugs due to some numbers being stored as objects (by-ref)
and thus needing special handling when cloning.

This leaves all the rest of the numbers as read-only.

Further simplified by representing globals size as a Smi.

BUG=

Review-Url: https://codereview.chromium.org/2381393002
Cr-Commit-Position: refs/heads/master@{#39923}