cc: Update SpreadForStdDeviation to 3*sigma, and update tests.

A recent change (https://codereview.chromium.org/1980613003) updated one path
to use 3*sigma as the estimated spread for gaussian blurs in filters. This
propagates that change to the "filter outset" path, for consistency, and
updates test results accordingly (including one visually-indistinguishable
pixel diff).

BUG=600821
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2004443002
Cr-Commit-Position: refs/heads/master@{#395126}
5 files changed