StringImpl.h should compile with -Wshorten-64-to-32
<http://webkit.org/b/114970>

Reviewed by Darin Adler.

Fixes the following warnings with -Wshorten-64-to-32:

    StringImpl.h:317:25: error: implicit conversion loses integer precision: 'uintptr_t' (aka 'unsigned long') to 'unsigned int' [-Werror,-Wshorten-64-to-32]
            unsigned hash = reinterpret_cast<uintptr_t>(this);
                     ~~~~   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

* wtf/text/StringImpl.h:
(WTF::StringImpl::StringImpl): Add static_cast<uint32_t>() to
formalize taking the lower 32-bits of the pointer value on
64-bit architectures.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148900 268f45cc-cd09-0410-ab3c-d52691b4dbfc