Fix bad scaling in TiledLayer

cc/layers/tiled_layer.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layers/tiled_layer.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "base/auto_reset.h"
@@ skipping 445 matching lines @@
 void TiledLayer::UpdateTileTextures(const gfx::Rect& update_rect,
                                     const gfx::Rect& paint_rect,
                                     int left,
                                     int top,
                                     int right,
                                     int bottom,
                                     ResourceUpdateQueue* queue,
                                     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
   // side effect of disabling compositing, which causes our reference to the
   // texture updater to be deleted.  However, we can't free the memory backing
   // the SkCanvas until the paint finishes, so we grab a local reference here to
   // hold the updater alive until the paint completes.
   scoped_refptr<LayerUpdater> protector(Updater());
   gfx::Rect painted_opaque_rect;
   Updater()->PrepareToUpdate(paint_rect,
@@ skipping 410 matching lines @@
   gfx::Rect prepaint_rect = visible_content_rect();
   prepaint_rect.Inset(-tiler_->tile_size().width() * kPrepaintColumns,
                       -tiler_->tile_size().height() * kPrepaintRows);
   gfx::Rect content_rect(content_bounds());
   prepaint_rect.Intersect(content_rect);
 
   return prepaint_rect;
 }
 
 }  // namespace cc