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 129ed07f6eb454e478f97327fc3e4969ea84065a..33011a39b7f1a223d0ee4955e1f1cf8b5fcacb1a 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -1568,6 +1568,11 @@ 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_);

[oshima, 2016/07/12 14:14:41]

physical_backing_size_ is resized in OnResize.

I think it's better to fold this method into OnResize, and then rename this to
OnSetDeviceScaleFactor. Update the size after OnDeviceScaleFactorChanged to keep
the order in sync with OnResize.

[wjmaclean, 2016/07/29 17:12:37]

I'm not sure I follow. By "fold" do you mean to put the code above this comment
into OnResize()?

When you say "update the size" do you mean the value of physical_backing _size_?
We cannot call OnResize() from here since we only have a new device scale
factor, and not an entire set of resize params. Finally, if we rename this to
OnSetDeviceScaleFactor, is that what the other current callers of this function
should call? If so, the logic they were expecting won't be here anymore.

[oshima, 2016/07/29 18:11:46]

Yes.
What I meant is to update & call stuff in the same order as OnResize.
It looks to me that OnResize is the only caller. Am I wrong?

 
   OnDeviceScaleFactorChanged();