https://bugs.webkit.org/show_bug.cgi?id=69018

       In addition to WTFReportBacktrace, this adds the cross-platform WTFGetBacktrace, which lets
       WebKit programmatically retrieve the current stack.  This is useful if you need to add more
       reporting to field crash report uploads, if you're tracking down an irreproducable bug,
       for instance.

       Reviewed by Darin Adler.

       * wtf/Assertions.cpp:
       * wtf/Assertions.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96595 268f45cc-cd09-0410-ab3c-d52691b4dbfc