CC: Have GPU-raster tiles exactly match the viewport.

cc/layers/picture_layer_impl_unittest.cc

 // Copyright 2013 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/picture_layer_impl.h"
 
 #include <algorithm>
 #include <limits>
 #include <set>
 #include <utility>
@@ skipping 4390 matching lines @@
   result = layer->CalculateTileSize(gfx::Size(42, 42));
   EXPECT_EQ(result.width(), 64);
   EXPECT_EQ(result.height(), 64);
   result = layer->CalculateTileSize(gfx::Size(184, 184));
   EXPECT_EQ(result.width(), 192);
   EXPECT_EQ(result.height(), 192);
   result = layer->CalculateTileSize(gfx::Size(199, 199));
   EXPECT_EQ(result.width(), 200);
   EXPECT_EQ(result.height(), 200);
 
+  // Gpu-rasterization uses 25% viewport-height tiles.
+  // The +2's below are for border texels.
   host_impl_.SetUseGpuRasterization(true);
-
-  // Gpu-rasterization uses 25% viewport-height tiles.
   result = layer->CalculateTileSize(gfx::Size(10000, 10000));
   EXPECT_EQ(result.width(), 1000);
-  EXPECT_EQ(result.height(), 250);
+  EXPECT_EQ(result.height(), 250 + 2);
 
   // Clamp and round-up, when smaller than viewport.
   result = layer->CalculateTileSize(gfx::Size(831, 10000));
   EXPECT_EQ(result.width(), 832);
-  EXPECT_EQ(result.height(), 250);
+  EXPECT_EQ(result.height(), 250 + 2);
 
   // Tile-height doubles to 50% when width shrinks to <= 50%.

[enne (OOO), 2014/10/10 00:06:05]

For what it's worth, I think this "viewport / 4" makes a lot of sense for long
scrollable layers.  But it doesn't make really all that much sense for most
other layers that (currently) get created.

I guess I'm trying to say this relative 50% is kind of weird and maybe should be
absolute.  Would it make sense to have a "max untiled size" for ganesh as well,
so that small layers don't get divided up by viewport size too?

   result = layer->CalculateTileSize(gfx::Size(447, 10000));
   EXPECT_EQ(result.width(), 448);
-  EXPECT_EQ(result.height(), 500);
+  EXPECT_EQ(result.height(), 500 + 2);
 
   // Largest layer is 50% of viewport width (rounded up), and
   // 50% of viewport in height.
   result = layer->CalculateTileSize(gfx::Size(447, 400));
   EXPECT_EQ(result.width(), 448);
   EXPECT_EQ(result.height(), 448);
   result = layer->CalculateTileSize(gfx::Size(500, 499));
   EXPECT_EQ(result.width(), 512);
-  EXPECT_EQ(result.height(), 500);
+  EXPECT_EQ(result.height(), 500 + 2);
 }
 
 }  // namespace
 }  // namespace cc