This patch adds required change for stackwalker_mips_unittest to pass on 64bit
platforms.

One test from stackwalker_mips_unittest was failing as mentioned in:
https://breakpad.appspot.com/632002/
https://breakpad.appspot.com/633002/

BUG=none
TEST=stackwalker_mips_unittest (run on x86-64 and mips)

Review URL: https://breakpad.appspot.com/634002/



git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1218 4c0a9323-5329-0410-9bdc-e9ce6186880e