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

Review URL: https://codereview.chromium.org/367143002

git-svn-id: svn://svn.chromium.org/blink/trunk@177412 bbb929c8-8fbe-4397-9dbb-9b2b20218538