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

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index 353c57676adbb5bf890bac1889cc90c2fb1d0693..7c9a4165eed9b4c478dfb681102c96e171577891 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -692,14 +692,13 @@ gfx::Size PictureLayerImpl::CalculateTileSize(
     int viewport_width = layer_tree_impl()->device_viewport_size().width();
     int viewport_height = layer_tree_impl()->device_viewport_size().height();
     default_tile_width = viewport_width;
-    default_tile_height = viewport_height / 4;
+    // Also, double the height when the content width falls below 50%, and
+    // pad by our border texels to make the tiles exactly match viewport.
+    int divisor = (content_bounds.width() <= viewport_width / 2) ? 2 : 4;
+    default_tile_height = RoundUp(viewport_height, divisor) / divisor;
+    default_tile_height += 2 * PictureLayerTiling::kBorderTexels;
     default_tile_height =
         std::max(default_tile_height, kMinHeightForGpuRasteredTile);
-
-    // Increase the tile-height proportionally when the content width
-    // drops below half the viewport width.
-    if (content_bounds.width() <= viewport_width / 2)
-      default_tile_height *= 2;
   } else {
     // For CPU rasterization we use tile-size settings.
     const LayerTreeSettings& settings = layer_tree_impl()->settings();