Compute time weighted median network quality estimate.

Compute median network quality estimate from the stored
observations. This is done by adding a general function
GetEstimate(int percentile).

Observations are assigned a weight based on their
timestamp. Recent observations are given higher weight.

BUG=472681

Review URL: https://codereview.chromium.org/1183203002

Cr-Commit-Position: refs/heads/master@{#335990}
7 files changed