Cross-process frames should be notified of device scale factor changes.

content/renderer/render_widget.cc

Index: content/renderer/render_widget.cc
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index 7f781534ca520bafea43659f771a87c611d589c8..7671068c6907883666dfb989991ba608fb94f41d 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -1578,6 +1578,11 @@ void RenderWidget::SetDeviceScaleFactor(float device_scale_factor) {
     return;
 
   device_scale_factor_ = device_scale_factor;
+  // TODO(wjmaclean): do we need the following line? It only seems to matter for
+  // InitializeLayerTreeView, which only happens during initialization in which
+  // case the physical_backing_size is (I think) set via the view params set
+  // during construction.
+  physical_backing_size_ = gfx::ScaleToCeiledSize(size_, device_scale_factor_);
 
   OnDeviceScaleFactorChanged();