Introduce JSFunction::EnsureHasInitialMap method.

Introduce JSFunction::EnsureHasInitialMap method.

This change enforces explicit allocation of the initial map for each
JSFunction to introduce a proper layering between the JSFunction class
and the Heap class. A follow-up change will then handlify the two
functions AllocateInitialMap and AllocateFunctionPrototype.

R=rossberg@chromium.org
BUG=v8:2877

Review URL: https://codereview.chromium.org/32323013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17480 ce2b1a6d-e550-0410-aec6-3dcde31c8c00