Fix GTestMac.AssertNSEQ on Mac 64.

Switch to NSStrings; NSNumbers may be tagged pointers.

BUG=244703
TEST=Mac 64 tester goes green

Review URL: https://chromiumcodereview.appspot.com/15690016

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@202944 4ff67af0-8c30-449e-8e8b-ad334ec8d88c