Removed a bunch of GetExistingThreadLocal calls by threading the Isolate.

Removed a bunch of GetExistingThreadLocal calls by threading the Isolate.

For Octane, the number of calls go down from 7341629 to 1947880, i.e. they are
reduced by more than 73%. TLS access is not especially cheap, so this exercise
seems worthwhile.

Review URL: https://codereview.chromium.org/11412007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12979 ce2b1a6d-e550-0410-aec6-3dcde31c8c00