Add a flag to d8 to invoke weak callbacks

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

Committed: https://code.google.com/p/v8/source/detail?r=21529

[Yang, 2014-05-27 13:42:04]

LGTM with comment.

https://codereview.chromium.org/306483006/diff/1/src/d8.cc
File src/d8.cc (right):

https://codereview.chromium.org/306483006/diff/1/src/d8.cc#newcode1387
src/d8.cc:1387: }
This duplicate code could be factored out into a helper function.

[jochen (gone - plz use gerrit), 2014-05-27 13:57:57]

Committed patchset #2 manually as r21529 (presubmit successful).