Valgrind cleanliness, part 1: Delete current isolate on exit.

Valgrind cleanliness, part 1: Delete current isolate on exit.

Currently we leak 140kB from 80 allocation sites when we evaluate a simple
"print(1+2)" in d8 and exit. This might not be a real problem in Chrome because
most of the time the whole process containing v8 vanishes, but it can hide real
leaks in a sea of false positives. Therefore, this CL and a few subsequent ones
clean this up a bit.

This simple CL alone fixes 45 leaks, leaving only 35 with a net loss of 2192
bytes (on ia32).

Review URL: https://chromiumcodereview.appspot.com/9791016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11146 ce2b1a6d-e550-0410-aec6-3dcde31c8c00