cc: Fix skewport math

cc/resources/picture_layer_tiling_unittest.cc

 // Copyright 2012 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/resources/picture_layer_tiling.h"
 
 #include <limits>
 #include <set>
 
 #include "cc/base/math_util.h"
@@ skipping 573 matching lines @@
       tiling->ComputeSkewport(1.5, gfx::Rect(-50, -50, 200, 200));
 
   // x and y moved by -75 (-50 - 75 = -125).
   // right side and bottom side moved by 75 [(350 - 125) - (200 - 50) = 75].
   EXPECT_EQ(-125, expand_skewport.x());
   EXPECT_EQ(-125, expand_skewport.y());
   EXPECT_EQ(350, expand_skewport.width());
   EXPECT_EQ(350, expand_skewport.height());
   EXPECT_TRUE(expand_skewport.Contains(gfx::Rect(-50, -50, 200, 200)));
 
-  // Expand the viewport past the limit.
+  // Expand the viewport past the limit in all directions.
   gfx::Rect big_expand_skewport =
       tiling->ComputeSkewport(1.5, gfx::Rect(-500, -500, 1500, 1500));
 
   EXPECT_EQ(-575, big_expand_skewport.x());
   EXPECT_EQ(-575, big_expand_skewport.y());
   EXPECT_EQ(1650, big_expand_skewport.width());
   EXPECT_EQ(1650, big_expand_skewport.height());
   EXPECT_TRUE(big_expand_skewport.Contains(gfx::Rect(-500, -500, 1500, 1500)));
+
+  // Shrink the skewport in all directions.
+  gfx::Rect shrink_viewport =
+      tiling->ComputeSkewport(1.5, gfx::Rect(0, 0, 100, 100));
+  EXPECT_EQ(0, shrink_viewport.x());
+  EXPECT_EQ(0, shrink_viewport.y());
+  EXPECT_EQ(100, shrink_viewport.width());
+  EXPECT_EQ(100, shrink_viewport.height());
+
+  // Move the skewport really far in one direction.
+  gfx::Rect move_skewport_far =
+      tiling->ComputeSkewport(1.5, gfx::Rect(0, 5000, 100, 100));
+  EXPECT_EQ(0, move_skewport_far.x());
+  EXPECT_EQ(5000, move_skewport_far.y());
+  EXPECT_EQ(100, move_skewport_far.width());
+  EXPECT_EQ(175, move_skewport_far.height());
+  EXPECT_TRUE(move_skewport_far.Contains(gfx::Rect(0, 5000, 100, 100)));
 }
 
 TEST(PictureLayerTilingTest, ComputeSkewport) {
   FakePictureLayerTilingClient client;
 
   gfx::Rect viewport(0, 0, 100, 100);
   gfx::Size layer_bounds(200, 200);
 
   client.SetTileSize(gfx::Size(100, 100));
   client.set_tree(ACTIVE_TREE);
@@ skipping 1346 matching lines @@
   tiling_->SetRasterSourceAndResize(pile);
 
   // Tile size in the tiling should be resized to 250x200.
   EXPECT_EQ(250, tiling_->TilingDataForTesting().max_texture_size().width());
   EXPECT_EQ(200, tiling_->TilingDataForTesting().max_texture_size().height());
   EXPECT_EQ(0u, tiling_->AllRefTilesForTesting().size());
 }
 
 }  // namespace
 }  // namespace cc