Swarming won't support an incremental data file. By just
sorting the lowest hanging fruits to the beginning we
already get a big bang for the buck (>80% of the improvement
we get otherwise).

This will require semi-regular manual updates of the
slowest tests.

BUG=chromium:535160
LOG=n

Review URL: https://codereview.chromium.org/1385443002

Cr-Commit-Position: refs/heads/master@{#31061}