Block mixed <picture> and <img srcset> by default.

Block mixed <picture> and <img srcset> by default.

It's going to be more or less impossible to begin blocking mixed <img>
content by default, given the amount of usage we're likely to see in the
wild (I'm still waiting for metrics from Chrome stable, but I'm not
hopeful). I think it will be significantly less impossible to avoid the
same mistake with the new responsive hotness.

This CL treats <picture> and <img srcset> as active content, blocking
them by default when mixed.

------------------------------------------------------------------------
This is a re-land of r177385, which was reverted because it broke a few
browser_tests, Chromium-side. This patch differs from the initial
patch[1] only in that it now starts with a copy of
'ResourceFetcher::defaultResourceLoaderOptions()' rather than an empty
ResourceLoaderOptions.

[1]: https://codereview.chromium.org/356243003
------------------------------------------------------------------------

BUG=390158
TBR=yoav@yoav.ws

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177464

[Mike West, 2014-07-03 09:32:22]

TBRing you, Yoav. Relanding this CL after fixing the ResourceLoaderOptions that
caused the revert yesterday.

-mike

[Mike West, 2014-07-03 09:32:25]

The CQ bit was checked by mkwst@chromium.org

[commit-bot: I haz the power, 2014-07-03 09:33:17]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mkwst@chromium.org/368203002/1

[commit-bot: I haz the power, 2014-07-03 10:40:39]

Change committed as 177464

[Yoav Weiss, 2014-07-03 10:58:10]

On 2014/07/03 10:40:39, I haz the power (commit-bot) wrote:
> Change committed as 177464

Retroactive 
LGTM :)