Improved srcset descriptor parsing.

Improved srcset descriptor parsing.

I've refactored srcset's descriptor parser in order to improve readability of the srcset parser, and improve spec compliance.

Changes:
* Descriptor parsing is now split to its own function.
* Invalid descriptors are ignored, rather than invalidate the candidate.
  - That includes the viewport qualifiers, which are not yet supported, but may be used by authors.
* Some unnecessary cruft has been removed from the srcset parser.
* Added tests that make sure descriptors are treated as they should.

BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=158167

[Yoav Weiss, 2013-09-22 12:46:47]

I did some refactoring to the srcset parser, to make sure it is spec compliant
and clean it up some more.

[abarth-chromium, 2013-09-22 16:08:36]

lgtm

[commit-bot: I haz the power, 2013-09-22 16:08:45]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/yoav@yoav.ws/23694049/1

[commit-bot: I haz the power, 2013-09-22 20:23:32]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/yoav@yoav.ws/23694049/1

[commit-bot: I haz the power, 2013-09-23 08:22:38]

Change committed as 158167