For the record.

Discussed dgarrett@ and shinyak@.
Now, in this bug, we focus on enabling DepsCache on chrome-pfq bots.

In chrome-pfq, we expect that a bot selected for a specific board was selected for the same configuration in previous run, in most cases.

It's future work to enable DepsCache on other bots,  crbug.com/751010 ,
because that assumption wouldn't work well in swarming era, probably. We need to investigate more later.

Forgot to set BUG= line...
Done in https://chromium-review.googlesource.com/c/559370.