Use an idle notification of 0ms as hint that a GC after context disposal is triggerd

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

Review URL: https://codereview.chromium.org/671513006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24773 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix data race with interface caching

R=bmeurer@chromium.org
BUG=421634
LOG=N

Review URL: https://codereview.chromium.org/667703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24774 ce2b1a6d-e550-0410-aec6-3dcde31c8c00