Don't allow style sharing between top layer and non-top layer elements

Normally, top layer elements are ensured a layer in adjustRenderStyle.
However, if style sharing occurs, the top layer element can receive a style
that causes it to not have a layer, which will break top layer rendering.

BUG=

Review URL: https://codereview.chromium.org/150273003

git-svn-id: svn://svn.chromium.org/blink/trunk@166215 bbb929c8-8fbe-4397-9dbb-9b2b20218538