Add fixed raster scale use counter histograms.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 545dfa30fe601b29f2c50c7a4b3c09e1ceee30d3..add6b0ffecf8daa252fa6f9c8df56d8208fd5268 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -557,6 +557,8 @@ bool CompositedLayerMapping::updateGraphicsLayerConfiguration()
 
     updateElementIdAndCompositorMutableProperties();
 
+    m_graphicsLayer->setHasWillChangeTransformHint(m_owningLayer.layoutObject()->styleRef().hasWillChangeTransformHint());

[chrishtr, 2016/05/05 20:33:07]

Can you do a quick test of a translateZ layer that adds and removes will-change,
and verify that it executes this line of code?
There are some dirty bits on the compositing code that I want to make sure this
works correctly with.

[vmpstr, 2016/05/06 19:53:48]

Yep, every will change update (with nothing else changing) hits this. 

+
     return layerConfigChanged;
 }