Add ratio function to calculate the ratio of two calculations.

To test this:

$ cd ~/golib/src/
$ go get -u -t skia.googlesource.com/buildbot.git/perf/go/...
$ cd skia.googlesource.com/buildbot.git/perf/
$ make testgo

BUG=None
TEST=see above
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/610793003
This adds http endpoints for correctness counts that are easily consumable by front-end JS.

One can request the counts of positive/negative/untriaged digests aggregated over tests and also for individual tests.

A second resource allows to request the counts for a specific test and also get labels for digests in the traces for the test.

BUG=skia:

Review URL: https://codereview.chromium.org/650253003
perf: Fix a typo in logserver_init script.

Add pass set up the log_dir to 'tmp/glog'.

BUG=None
TEST=None
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/664553005
Use metrics.Counter for filediffstore.downloadFailureCount

Addressing comment in https://codereview.chromium.org/625863003/#msg14

BUG=skia:

Review URL: https://codereview.chromium.org/641813004