Fix heap-use-after-free in GraphicsLayer::setContentsTo()

GraphicsLayer only holds a reference of contents layer pointer in
m_contentsLayer, but the object owning the layer may destroy it without
clearing GraphicsLayer::m_contentsLayer. m_contentsLayer cann't be accessed
after the owning object destroy the layer. In addition, during
destroying the layer, WebLayerImpl::web_layer_client_is cleared, so
setWebLayerClient() doesn't need to be called.

BUG=273732

Review URL: https://chromiumcodereview.appspot.com/22875020

git-svn-id: svn://svn.chromium.org/blink/trunk@156194 bbb929c8-8fbe-4397-9dbb-9b2b20218538