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.

[Stephen Chennney, 2016/10/17 14:31:30]

"... is the size of each instance of the image, in pixels."

[chrishtr, 2016/10/19 03:51:36]

Isn't it in destination space though? So it isn't equal to the intrinsic image
size.

[Stephen Chennney, 2016/10/19 14:48:15]

Right, my comment wasn't clear either. I also realize that this size has meaning
even if we are not tiling. Is this better? "tileSize() defines the size for the
background image to meet CSS background-size and background-repeat
requirements."

[chrishtr, 2016/10/19 16:56:07]

I added a sentence referencing CSS as an example, and changed a few other words.
Hope that is clearer.

+  //
+  // 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.

[Stephen Chennney, 2016/10/19 14:48:15]

Then this comment stays to make it clear we will resize to the tileSize.

[chrishtr, 2016/10/19 16:56:07]

Ack.

+  //
+  // spaceSize().width in the horizontal and spaceSize().height in the vertical
+  // direction are the spaces between tiles, if the image is tiled.

[Stephen Chennney, 2016/10/19 14:48:15]

There's already a comment below on spaceSize(). I don't think we need this new
one.

[chrishtr, 2016/10/19 16:56:06]

Removed.

   const LayoutRect& destRect() const { return m_destRect; }
   const LayoutSize& tileSize() const { return m_tileSize; }
   const LayoutPoint& phase() const { return m_phase; }

[Stephen Chennney, 2016/10/19 14:48:15]

If we want to comment all of them, this one is "phase() represents the point in
the image that will appear at (0,0) in the destination space. The point is
defined in tileSize() coordinates."

[chrishtr, 2016/10/19 16:56:06]

Done.