Add buildbot data ingestion to datahopper

BUG=skia:

Review URL: https://codereview.chromium.org/716183002
Remove now-unused slaves from slave_hosts_cfg.py

BUG=skia:

Review URL: https://codereview.chromium.org/720243002
Add the ability to query across traces.
This should be the last missing key feature.

BUG=skia:2930

Review URL: https://codereview.chromium.org/713323003
Move skiaperf to Polymer 0.5.1

BUG=skia:

Review URL: https://codereview.chromium.org/723963005