Make RenderFrameProxyHost::GetRenderViewHost return its own RVH.

RFPH has a render_view_host_ member, introduced in
https://crrev.com/c/1665936 when RVH was made refcounted but that is not
what GetRenderViewHost returns. Instead it pulls it from the frame tree.

Patchset 1 of this CL (https://crrev.com/c/2332109/1) shows that these
values never differ currently, so we can use the simpler (and less
surprising) version.

Change-Id: I2046afd17c66785c63e151f546682c7babf50925
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332109Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Auto-Submit: Fergal Daly <fergal@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794207}

content/browser/frame_host/render_frame_proxy_host.cc

@@ -172,9 +172,7 @@ void RenderFrameProxyHost::SetChildRWHView(
 }
 
 RenderViewHostImpl* RenderFrameProxyHost::GetRenderViewHost() {
-  return frame_tree_node_->frame_tree()
-      ->GetRenderViewHost(site_instance_.get())
-      .get();
+  return render_view_host_.get();
 }
 
 RenderWidgetHostView* RenderFrameProxyHost::GetRenderWidgetHostView() {