Picture element initial implementation

Picture element initial implementation

/* This is only here for historic reasons :) This patch is no longer relevant in any way */

BUG=http://crbug.com/233751

Spec=http://www.w3.org/TR/html-picture-element/

Enable a responsive images solution by giving developers control over the image resource that is downloaded & displayed using multiple resources, each with its own media attributes. 

This implementation is in its initial phase, and doesn't yet represent a complete implementation of the specification.

[Yoav Weiss, 2013-04-23 22:57:16]

Following
https://groups.google.com/a/chromium.org/forum/#!topicsearchin/blink-dev/subj...
and despite the NOT LGTM decision on the feature's implementation, I would
highly appreciate if this patch can be reviewed.

Thanks!