Support x86_64 Mac for nacl_integration_tests.

This enables nacl_integration_tests to run via buildbot_nacl_integration.py
for Mac x86_64. It makes --bits work correctly on Mac, and allows the proper
--bits setting to be automatically detected based on the Chrome browser being
used for testing.

BUG=316401,316715
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/67313002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234035 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234035
Blink roll 161630:161635

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=161631:161635&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/66193006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234036 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234036