Remove <video controls> hack.

insertedInto and removedFrom should always be paired, there's no
reason to track this state separately when updating the childShadowRoot
bits.

I also removed the inDocument() checks from ShadowRoot::insertedInto
and removed from since distribution happens in detached subtrees too.
It appears that while they were wrong, it didn't matter because the
state was only used together with containsContentElements() in a check
of the form:

if (!containsShadowRoots() && !containsContentElements())
  return;

which meant that while the containsShadowRoots bit might be wrong false
incorrectly, the containsContentElements is always correct so we'd
never take the early out incorrectly.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/801643009