Reduce heavy runtime calls from debug mirrors.

Cache results of some heavy calls into C++.
This alone will boost the speed up to 18% on some real world scenarios.

BUG=chromium:340285
LOG=Y
R=ulan@chromium.org, yangguo@chromium.org, Yang, rossberg, ulan, yurys

Review URL: https://codereview.chromium.org/184483004

Patch from Andrey Adaykin <aandrey@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19714 ce2b1a6d-e550-0410-aec6-3dcde31c8c00