Replace check for RenderLayer in RenderLayerModelObject::ensureLayer with ASSERT due to redundant check.

RenderLayerModelObject::styleDidChange() invokes the ensureLayer() method only in case of !layer().
Again checking for layer existence would be deemed redundant in ensureLayer. Replacing it with an
ASSERT(!m_layer).

Review URL: https://codereview.chromium.org/27246003

git-svn-id: svn://svn.chromium.org/blink/trunk@159776 bbb929c8-8fbe-4397-9dbb-9b2b20218538