Make ToBooleanStub more consistent across platforms.

Make ToBooleanStub more consistent across platforms.

The declaration of the ToBoolean class moved to the platform-independent part
and its implementations are now structurally very similar. This is just an
intermediate cleanup step to add type recording at the call site.

Note that the MIPS implementation has not really been touched, so it should
continue to work, too.
Review URL: http://codereview.chromium.org/7218012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8359 ce2b1a6d-e550-0410-aec6-3dcde31c8c00