ROBUST_JPEG_CODEC was added in the past because using the default
decoder, that accepts various image formats, in login screens
is potentially dangerous.

However, JPEG format does not support transparent pixels, hence
profile images with transparent pixels are rendered incorrectly.

Per discussion with jorgelo@ and mdempsky@, this patch introduces
ROBUST_PNG_CODEC, that accepts only PNG data on top of the libpng
decoder.

BUG=587289
TEST=run the newly added browser tests

Review URL: https://codereview.chromium.org/1764603002

Cr-Commit-Position: refs/heads/master@{#380086}