Security drop fullscreen for any nested WebContents level.

This relands 3dcaec6e30feebefc11e with a fix to the test.

BUG=873080
TEST=as in bug

Change-Id: Ie68b197fc6b92447e9633f233354a68fefcf20c7
Reviewed-on: https://chromium-review.googlesource.com/1175925
Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583335}
 void WebContentsImpl::DidCallFocus() {
  ForSecurityDropFullscreen();
 }

 void WebContentsImpl::DidCallFocus() {
  if (IsFullscreenForCurrentTab())
    ExitFullscreen(true);
 }
