Initialize Date parse cache with SMI instead of double to workaround sharing mutable heap numbers in snapshot.

This is the only field in the snapshot that was tracked as double.

R=verwaest@chromium.org
TEST=mjsunit/regress/regress-280531.js
BUG=280531
LOG=Y

Review URL: https://chromiumcodereview.appspot.com/112003005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18298 ce2b1a6d-e550-0410-aec6-3dcde31c8c00