Introduce object visitor to estimate the size of a native context.

Introduce object visitor to estimate the size of a native context.

This is only an estimate since it counts objects that could be shared,
for example strings, cow arrays, heap numbers, etc.

It however ignores objects that could be shared, but may only be used
by the context to be measured, for example shared function infos,
script objects, scope infos, etc.

R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1268333004

Cr-Commit-Position: refs/heads/master@{#30029}