Move WebCore/platform into its own Platform namespace
This will help detect and prevent layering violations, as using WebCore:: from within platform/ will be disallowed.
This is an incremental step towards moving Platform into its own separate module, as discussed in:
Based on IRC discussion it might be useful to move code into Source/Platform but leave it in the WebCore namespace. Having a separate directory and include paths will let us enforce strict dependency ordering, as code in Source/Platform/ will not have any paths in Source/WebCore/* on its include path, but leaving it in the WebCore namespace can ease the transition. Retitling bug to be more general (even if we change the namespaces of things we will definitely also change the directory).