If a context snapshot includes the global proxy constructor function, we
expect the incoming global proxy to have the correct instance size so
that we can reinitialize it with said constructor. However, when the
bootstrapper allocates a new global proxy, we need to know the expected
size.

We solve this by storing the size on the to-be-serialized isolate.

R=jochen@chromium.org, peria@chromium.org
BUG=chromium:617892

Review-Url: https://codereview.chromium.org/2585693002
Cr-Commit-Position: refs/heads/master@{#41756}