Cleanup run_default_swarm_tests usage.
Cleanup run_default_swarm_tests usage.

The previous implementation had layering violation. Cleans up the builder
factory to be more readable. Comment about why chromium_swarm_tests is being
forcibly added.

R=iannucci@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/14924003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@198548 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build