Add picture related attributes to Source

As part of the picture specification, the source element gets added with several new attributes, that are used in HTMLImageElement's source selection algorithm.
This CL adds these attributes.

BUG=368830

Review URL: https://codereview.chromium.org/267693002

git-svn-id: svn://svn.chromium.org/blink/trunk@173727 bbb929c8-8fbe-4397-9dbb-9b2b20218538