With this, the test runner automatically merges sancov
files after testing. There's no need to do this by some
external infrastructure.

In a future CL, we could even merge during testing to lift
harddisk pressure.

BUG=chromium:568949
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1776123002

Cr-Commit-Position: refs/heads/master@{#34606}