Add a flag to d8 to invoke weak callbacks

This will send an idle notification and a low memory notification after
each test.

For some reason it's not enough to send a low memory notification alone.

BUG=none
R=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/306483006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21529 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Drop unused includes from worker-thread.cc

BUG=none
R=marja@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/302463007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21530 ce2b1a6d-e550-0410-aec6-3dcde31c8c00