Remove usages of WebContents::GetRenderViewHost().

This CL was uploaded by git cl split.

R=sky@chromium.org

Bug: 1142671
Change-Id: Ied44b2bee60ee3724ec79693f74d857e9a48d5a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500577
Auto-Submit: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821446}

ui/views/controls/webview/web_contents_set_background_color.cc

@@ -35,6 +35,7 @@ WebContentsSetBackgroundColor::~WebContentsSetBackgroundColor() = default;
 
 void WebContentsSetBackgroundColor::RenderViewReady() {
   web_contents()
+      ->GetMainFrame()
       ->GetRenderViewHost()
       ->GetWidget()
       ->GetView()