Make top layer elements occlude plugins.

Before this patch, native plugins such as Flash would always be rendered
above top layer elements.  This patch adds top layer elements to the
cutout rects for a plugin, that were already generated for the "iframe
shim" mechanism.

This patch requires manual tests because content_shell doesn't seem to
handle plugins well (crbug.com/285780). As a result, a conventional
LayoutTest would pass with or without this patch.

This patch does not yet correctly handle interaction between iframes and
top layer when occluding the plugin. For example, an iframe will still
occlude a plugin in the top layer, since the existing code for only
examines zindex, not top layer.

Another flaw is that the plugin cannot be shown under an element with
a translucent background; the plugin is completely occluded.

BUG=285775

Review URL: https://chromiumcodereview.appspot.com/23494031

git-svn-id: svn://svn.chromium.org/blink/trunk@158001 bbb929c8-8fbe-4397-9dbb-9b2b20218538