V8 zaps (writes 0xdeadbeef) over the mmapped regions when in debug mode.
This causes more resident size than displayed in tracing. So, This CL
adds an api to tell if zapping is done.

BUG=546492
LOG=Y

Review URL: https://codereview.chromium.org/1419523008

Cr-Commit-Position: refs/heads/master@{#31716}