Revert 177385 "Block mixed <picture> and <img srcset> by default."

Revert 177385 "Block mixed <picture> and <img srcset> by default."

Seems to have broken some cross-origin browser_tests.

BUG=391027, 390158
> 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.
> 
> BUG=390158
> 
> Review URL: https://codereview.chromium.org/356243003

TBR=mkwst@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=177412

[Ken Russell (switch to Gerrit), 2014-07-02 19:13:11]

Committed patchset #1 manually as r177412.