Add compression regression collector.

This tests hits a test page and
computes the compression rate for each of the resource types on it.
These metrics are then output to the console. After metrics from several
releases have been manually captured from the console, his test be
changed to check compression rates against the known good rates, and
alert if they fall outside a tolerance.

BUG=687216

Review-Url: https://codereview.chromium.org/2666963002
Cr-Commit-Position: refs/heads/master@{#447860}