Fix d8's Shell::ReadBuffer after r18227

BUG=v8:3085
LOG=N
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/127853003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18482 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix selection of popular pages in store buffer.

BUG=331444
TEST=mjsunit/regress/regress-331444.js
LOG=Y
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/125983002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18483 ce2b1a6d-e550-0410-aec6-3dcde31c8c00