[mac] Encode layout test result images as RGB png

[mac] Encode layout test result images as RGB png

As discussed on issue 409435, mac png test results are RGBA encoded
while other ports use RGB encoded pngs and that for compat with the
mac webkit port's ImageDiff tool, so that layout test pixel results
also worked for the Safari webkit port.

The compat reasons are no longer an issue. Make the mac test driver
encode layout test result images as RGB png. All new rebaselines on
mac will be written as RGB png from here on, and, per issue 409435,
will match the win results and thus be optimized away.

CQ_EXTRA_TRYBOTS=tryserver.blink:mac_blink_rel,mac_blink_dbg
BUG=409435, 376232

Review URL: https://codereview.chromium.org/545823002

Cr-Commit-Position: refs/heads/master@{#303144}