Fix bad scaling in TiledLayer

cc/layers/tiled_layer.cc

Index: cc/layers/tiled_layer.cc
diff --git a/cc/layers/tiled_layer.cc b/cc/layers/tiled_layer.cc
index 4a960a84f5261e2b4ff0fb4db8c079fb9f181dfa..0af1aa68f4eaad91ff27d63017ece600f450bf57 100644
--- a/cc/layers/tiled_layer.cc
+++ b/cc/layers/tiled_layer.cc
@@ -463,12 +463,8 @@ void TiledLayer::UpdateTileTextures(const gfx::Rect& update_rect,
                                     const OcclusionTracker<Layer>* occlusion) {
   // The update_rect should be in layer space. So we have to convert the
   // paint_rect from content space to layer space.
-  float width_scale =
-      paint_properties().bounds.width() /
-      static_cast<float>(content_bounds().width());
-  float height_scale =
-      paint_properties().bounds.height() /
-      static_cast<float>(content_bounds().height());
+  float width_scale = 1 / draw_properties().contents_scale_x;

[danakj, 2014/09/13 13:04:50]

Ok ya, this would be correct, but you need to fill that last row/column of
pixels with something. Otherwise if the content doesn't fill the entire bitmap,
it'll be blending that last pixel with transparent or uninitialized memory.

This is where we do the same thing in impl-side painting:
https://code.google.com/p/chromium/codesearch#chromium/src/cc/resources/pictu...

Previously we dealt with that by scaling to fill the entire bitmap instead of
using the exact scale, and we wrote tests for that, so the tests will need to be
updated.

+  float height_scale = 1 / draw_properties().contents_scale_y;
   update_rect_ = gfx::ScaleRect(update_rect, width_scale, height_scale);
 
   // Calling PrepareToUpdate() calls into WebKit to paint, which may have the