Blink Compositor: Merge initializeLayerTreeView and layerTreeView.

In WebWidgetClient.

This is a preparation CL to move cc::LayerTreeHost ownership from
content::RenderWidgetCompositor to Blink.

BUG=584551

Review-Url: https://codereview.chromium.org/2594913003
Cr-Commit-Position: refs/heads/master@{#442523}
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index 59b1d939..87b5077 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -1222,7 +1222,7 @@
     compositor_->setViewportSize(physical_backing_size_);
 }
 
-void RenderWidget::initializeLayerTreeView() {
+blink::WebLayerTreeView* RenderWidget::initializeLayerTreeView() {
   DCHECK(!host_closing_);
 
   compositor_ = RenderWidgetCompositor::Create(this, device_scale_factor_,
@@ -1239,6 +1239,8 @@
   DCHECK_NE(MSG_ROUTING_NONE, routing_id_);
   compositor_->SetFrameSinkId(
       cc::FrameSinkId(RenderThread::Get()->GetClientId(), routing_id_));
+
+  return compositor_.get();
 }
 
 void RenderWidget::WillCloseLayerTreeView() {
@@ -1254,10 +1256,6 @@
     widget->willCloseLayerTreeView();
 }
 
-blink::WebLayerTreeView* RenderWidget::layerTreeView() {
-  return compositor_.get();
-}
-
 void RenderWidget::didMeaningfulLayout(blink::WebMeaningfulLayout layout_type) {
   if (layout_type == blink::WebMeaningfulLayout::VisuallyNonEmpty) {
     QueueMessage(new ViewHostMsg_DidFirstVisuallyNonEmptyPaint(routing_id_),