Buildbot dashboard: space builds by time
Buildbot dashboard: space builds by time

This makes more visual sense: rather than piling all builds against the
left side, builds are spaced in order of execution, ie. one per column
for each builder/buildslave combo.

Before: https://screenshot.googleplex.com/19mMb9uz4ms.png
After:  https://screenshot.googleplex.com/Q9ntKMSX4au.png

BUG=skia:3835

Review URL: https://codereview.chromium.org/1153193005

google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot