Only zapped used handles if a handle scope fits entirely within one handle block

Only zapped used handles if a handle scope fits entirely within one handle block

This should avoid zapping the same unused handles over and over again
when using many small nested handle scopes.

BUG=none
R=mstarzinger@chromium.org, vegorov@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=16987

[Michael Starzinger, 2013-09-27 11:24:30]

LGTM.

https://codereview.chromium.org/24359003/diff/1/src/handles-inl.h
File src/handles-inl.h (right):

https://codereview.chromium.org/24359003/diff/1/src/handles-inl.h#newcode133
src/handles-inl.h:133: std::swap(current->next,  prev_next);
nit: Two white-spaces after comma.

[jochen (gone - plz use gerrit), 2013-09-27 11:31:13]

Committed patchset #1 manually as r16987 (presubmit successful).