Remove null check in RenderViewImpl::ApplyPageVisibility.

Why would the browser be changing the page visibility after it has
detached the main frame, closing the RenderWidget and destroying the
WebView? It wouldn't.

R=dcheng@chromium.org

Change-Id: Ibc49c8b9ffbcf174819d88423125ca2e47c775d8
Bug: 894899
Reviewed-on: https://chromium-review.googlesource.com/c/1323698
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606170}

content/renderer/render_view_impl.cc

@@ -1872,8 +1872,6 @@ void RenderViewImpl::UpdateZoomLevel(double zoom_level) {
 void RenderViewImpl::ApplyPageVisibility(
     blink::mojom::PageVisibilityState visibility_state,
     bool initial_setting) {
-  if (!webview())
-    return;
   webview()->SetVisibilityState(visibility_state, initial_setting);
   // Note: RenderWidget visibility is controlled independently by the browser,
   // so there's no need to set visibility on the main frame's RenderWidget (or