cc : Reland move screen space scale factor to root transform node

third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp

Index: third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
index c74657e3ba39534a300335674b14ab1b33f9e895..5685dda8caa5ac94df65596cf1797908d7f68be6 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
@@ -453,8 +453,17 @@ class PropertyTreeManager {
 };
 
 void PropertyTreeManager::setDeviceScaleFactor(float deviceScaleFactor) {
-  auto& rootTransformNode = *transformTree().Node(kSecondaryRootNodeId);
-  rootTransformNode.local.Scale(deviceScaleFactor, deviceScaleFactor);
+  // TODO(jaydasika): We shouldn't set ToScreeen and FromScreen of root
+  // transform

[pdr., 2016/11/02 20:27:25]

Nit: the comment wrapping is a little weird here. Please rewrap to something
like:

// TODO(jaydasika): We shouldn't set ToScreeen and FromScreen of root
// transform node here. They should be set while updating transform
// tree in cc.

[jaydasika, 2016/11/02 20:45:47]

Done.

+  // node here. They should be set while updating transform tree in cc.
+  gfx::Transform toScreen;
+  toScreen.Scale(deviceScaleFactor, deviceScaleFactor);
+  transformTree().SetToScreen(kRealRootNodeId, toScreen);
+  gfx::Transform fromScreen;
+  bool invertible = toScreen.GetInverse(&fromScreen);
+  DCHECK(invertible);

[pdr., 2016/11/02 20:27:25]

Why is invertible required now but wasn't in your previous patch?

Do we need to update ancestors_are_invertible?

[jaydasika, 2016/11/02 20:45:47]

In the previous patch, it was
if (!toScreen.GetInverse(to_screen)
  ancestors_are_invertible = false
I realized that since its just a scale matrix, inverse will always exist and so
I changed it to DCHECK (ancestors_are_invertible is initliazed to true so we
don't have to explicitly set it to true here).

+  transformTree().SetFromScreen(kRealRootNodeId, fromScreen);
+  transformTree().set_needs_update(true);
 }
 
 int PropertyTreeManager::compositorIdForTransformNode(