Drop dependency on Isolate* from platform.h

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

Review URL: https://codereview.chromium.org/328993003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21819 ce2b1a6d-e550-0410-aec6-3dcde31c8c00