Fix WTF::copyLCharsFromUCharSource() to compile with -Wshorten-64-to-32

Fix WTF::copyLCharsFromUCharSource() to compile with -Wshorten-64-to-32
<http://webkit.org/b/107227>

Reviewed by Benjamin Poulain.

Fixes the following build error:

    ASCIIFastPath.h:117:59: error: implicit conversion loses integer precision: 'unsigned long' to 'const unsigned int' [-Werror,-Wshorten-64-to-32]
            const unsigned endLength = length - ucharsPerLoop + 1;
                           ~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~^~~
    1 error generated.

* wtf/text/ASCIIFastPath.h:
(WTF::copyLCharsFromUCharSource): Change local variables from
unsigned to size_t.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@140162 268f45cc-cd09-0410-ab3c-d52691b4dbfc