Fix memory usage reporting in performance_ui_tests
Fix memory usage reporting in performance_ui_tests

rsz is not supported on OS X, but rss is. This
causes performance_ui_tests to report more reliable
memory usage statistics.

Review URL: http://codereview.chromium.org/8231016

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@104991 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,ropik/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,adobe/chromium,yitian134/chromium