Fix warnings on Win64.

Fix warnings on Win64.
Set warning level to /W3 and change implicit conversions from size_t
to int. Most "fixes" are simply manifesting the implicit casts or using
a special strlen replacement that returns int.

Review URL: http://codereview.chromium.org/390004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3273 ce2b1a6d-e550-0410-aec6-3dcde31c8c00