Use request start time for estimating network quality.

Instead of using request creation time, use request start
time from load timing info. The start time is slightly
later than request creation time and is more accurate.

BUG=472681

Review URL: https://codereview.chromium.org/1162293004

Cr-Commit-Position: refs/heads/master@{#334499}
4 files changed