Revert "Allow two heartbeat to run simultaneously on the same bot."
Revert "Allow two heartbeat to run simultaneously on the same bot."

This reverts commit 04a9389ed0d068ba707ed6882e93a6af358376e1.

It makes things worse; now two builds can occur on one builder but not a build
on the two builders simultaneously. Oh well.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/619123003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@292263 0039d316-1c4b-4281-b951-d872f2087c98

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