The old HashMap class had an explicit member to determine the allocation
policy. The template version matches the approach used already for
lists.

Cleanup some include dependencies and unnecessary forward declarations.

Cleanup some dead code from isolate.h and replace some HEAP macros
with GetHeap().
Review URL: https://chromiumcodereview.appspot.com/9372106

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10806 ce2b1a6d-e550-0410-aec6-3dcde31c8c00