Fix two srcset related bugs

Fix two srcset related bugs

Two problems with the current srcset parsing algorithm are fixed here:
1. When a set of descriptors contained only invalid descriptors, the resource wasn't ignored.
2. When more than 1 resource shared a single DPR value, the last one was taken into account, rather than the first one.

I've also added test and fixed current ones to prevent this from regressing in the future.

BUG=347998
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168241

[Yoav Weiss, 2014-02-28 15:46:52]

Following the beta release I've got a bug report on twitter
(https://twitter.com/benfosterdev/status/439365333372391424) with an example.
(ain't shipping great :) )

Looking into it, I've found 2 bugs in the current srcset implementation. This CL
fixes these issues.

[abarth-chromium, 2014-03-01 06:08:51]

LGTM

Is this issue important enough to warrant merging to the beta branch?

[abarth-chromium, 2014-03-01 06:08:56]

The CQ bit was checked by abarth@chromium.org

[commit-bot: I haz the power, 2014-03-01 06:09:11]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/yoav@yoav.ws/184523003/20001

[Yoav Weiss, 2014-03-01 06:16:09]

On 2014/03/01 06:08:51, abarth wrote:
> LGTM
> 
> Is this issue important enough to warrant merging to the beta branch?

I believe it is. Otherwise, people experimenting with the srcset syntax (with
either invalid/future descriptors or with having the 1x resource defined in both
src and srcset while page is zoomed) will get unexpected results.

[abarth-chromium, 2014-03-01 07:09:02]

On 2014/03/01 06:16:09, Yoav Weiss wrote:
> I believe it is. Otherwise, people experimenting with the srcset syntax (with
> either invalid/future descriptors or with having the 1x resource defined in
both
> src and srcset while page is zoomed) will get unexpected results.

Ok.  You should add the Merge-Requested label to the associated bug once this
goes out to Canary.

[commit-bot: I haz the power, 2014-03-01 08:47:40]

Change committed as 168241