Plumb preferred raster scale for background images from Blink to cc layers.

third_party/WebKit/Source/core/paint/BackgroundImageGeometry.h

Index: third_party/WebKit/Source/core/paint/BackgroundImageGeometry.h
diff --git a/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.h b/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.h
index 1215ad22cbd1ca195b91a85383b1f34795cfcb08..b2f932cb4d4d9c667234eccf0b904b3b913064a1 100644
--- a/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.h
+++ b/third_party/WebKit/Source/core/paint/BackgroundImageGeometry.h
@@ -30,6 +30,16 @@ class BackgroundImageGeometry {
                  const FillLayer&,
                  const LayoutRect& paintRect);
 
+  // destRect() is the rect in the space of the containing box into which to
+  // draw the image. If the image is repeated via tiling, tileSize() is the size
+  // in pixels of the box to draw the image once.
+  //
+  // tileSize() need not be the same as the intrinsic size of the image; if not,
+  // it means the image will be resized (via an image filter) when painted into
+  // that tile region.
+  //
+  // spaceSize().width in the horizontal and spaceSize().height in the vertical
+  // direction are the spaces between tiles, if the image is tiled.
   const LayoutRect& destRect() const { return m_destRect; }
   const LayoutSize& tileSize() const { return m_tileSize; }
   const LayoutPoint& phase() const { return m_phase; }