Only send RenderWidget waterfall properties to child RenderWidgets

The main frame RenderWidget has page scale and pinch zoom determined
in the renderer by blink. Only child frames hear about it through
the VisualProperties waterfall propagation down the RenderWidget tree.

So stop explicitly sending (default) values to the main frame
RenderWidget (which it will not use) from RenderWidgetHostImpl.

R=avi@chromium.org, wjmaclean@chromium.org

Bug: 419087
Change-Id: I78a03df4d82cd1bb3765db8dffd92ea0e8bed812
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865530
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707364}

content/browser/renderer_host/render_widget_host_impl.cc

@@ -847,11 +847,11 @@ VisualProperties RenderWidgetHostImpl::GetVisualProperties() {
   // 7. Browser:    parent RenderWidgetHost (We're here if |is_child_frame|.)
   // 8. IPC           -> WidgetMsg_UpdateVisualProperties
   // 9. Renderer B: child  RenderWidget
-  visual_properties.page_scale_factor =
-      properties_from_parent_frame_.page_scale_factor;
-  visual_properties.is_pinch_gesture_active =
-      properties_from_parent_frame_.is_pinch_gesture_active;
   if (is_child_frame) {
+    visual_properties.page_scale_factor =
+        properties_from_parent_frame_.page_scale_factor;
+    visual_properties.is_pinch_gesture_active =
+        properties_from_parent_frame_.is_pinch_gesture_active;
     visual_properties.compositor_viewport_pixel_rect =
         properties_from_parent_frame_.compositor_viewport;