Ignore ConfusingAndOftenMisusedDisallowShadowContent

The only test that fails when you remove this seems to be the one that was
added when the flag was introduced: https://bugs.webkit.org/show_bug.cgi?id=80847

Formerly we'd always go out of the shadow root when doing a hit test, which
seems to relate to some sketchy checks in EventHandler that assume when you
click an <input> that the hit test never returns an element inside it.

Seeing as no tests fail though, I think we should just remove this and then
beat down the bugs it exposes in EventHandler.cpp

BUG=386967

Review URL: https://codereview.chromium.org/337373009

git-svn-id: svn://svn.chromium.org/blink/trunk@176900 bbb929c8-8fbe-4397-9dbb-9b2b20218538