Fix callsite of RenderWidgetHostImpl::is_focused()

Looking at the callsites of SetPageFocus, we only set is_focused for
main frame RWHs.

Change-Id: Ib28e28ea83d4815469484c509921dcde4143135c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2611945Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842062}

content/browser/renderer_host/render_frame_host_impl.cc

@@ -6985,7 +6985,8 @@ void RenderFrameHostImpl::InvalidateMojoConnection() {
 }
 
 bool RenderFrameHostImpl::IsFocused() {
-  if (!GetRenderWidgetHost()->is_focused() || !frame_tree_->GetFocusedFrame())
+  if (!GetMainFrame()->GetRenderWidgetHost()->is_focused() ||
+      !frame_tree_->GetFocusedFrame())
     return false;
 
   RenderFrameHostImpl* focused_rfh =