<body> overflow clip still applies after being removed

The logic after https://codereview.chromium.org/87443002
wouldn't clear the overflow clip bit in every code path.
This would basically leave an overflow clip when the author
removed it. In order to prevent further regression, this change
simplify the nested conditions that made it hard to see when
the overflow clip was actually set or reset.

BUG=326962

Review URL: https://codereview.chromium.org/126063002

git-svn-id: svn://svn.chromium.org/blink/trunk@164675 bbb929c8-8fbe-4397-9dbb-9b2b20218538
HTMLSelectElement should not override childShouldCreateRenderer

Instead the two renderers it creates should reject children with
isChildAllowed(). Even that's a hack though, we should use a ShadowRoot
with insertion points in the future but we can't do that yet because
too many things break.

This and https://codereview.chromium.org/125203004/ leave only
HTMLMediaElement overriding childShouldCreateRenderer outside of
SVG. A future patch will remove that override too.

Review URL: https://codereview.chromium.org/118723006

git-svn-id: svn://svn.chromium.org/blink/trunk@164676 bbb929c8-8fbe-4397-9dbb-9b2b20218538