cc: Make tiling interest rect calc based on viewport.

cc/layers/picture_layer_impl_unittest.cc

Index: cc/layers/picture_layer_impl_unittest.cc
diff --git a/cc/layers/picture_layer_impl_unittest.cc b/cc/layers/picture_layer_impl_unittest.cc
index f5702d32614d280e6e6bdbafdcf429166d17d2c5..6d7d622c0899bed0cc5f409197d2303a072f449e 100644
--- a/cc/layers/picture_layer_impl_unittest.cc
+++ b/cc/layers/picture_layer_impl_unittest.cc
@@ -2555,8 +2555,8 @@ TEST_F(PictureLayerImplTest, RequiredTilesWithGpuRasterization) {
 
   active_layer_->HighResTiling()->UpdateAllTilePrioritiesForTesting();
 
-  // High res tiling should have 64 tiles (4x16 tile grid).
-  EXPECT_EQ(64u, active_layer_->HighResTiling()->AllTilesForTesting().size());
+  // High res tiling should have 36 tiles (4x9 tile grid).

[vmpstr, 2015/05/06 17:57:29]

Can you verify manually that it is actually 4x9? I buy that there's less tiles
due to a smaller interest area, but I'm not sure I buy 4x9...

[sohanjg, 2015/05/07 14:42:10]

Hmm..for this test viewport is 1kX1k, tile size is 256X1002, so each viewport =
3.89 tiles (4, roughly), we have 9 viewport worth of interest area.

[vmpstr, 2015/05/07 17:21:38]

There should be exactly 4 tiles in the viewport when accounting for border
pixels. Aside from that 4x9 doesn't refer to 4 tiles/viewport times 9 viewports.
4x9 says that there are 4 tiles across the tiling, and 9 tiles down, which is
what I don't really buy.

I think it might be 3x12 or something like that. Can you just print out all of
the tile indices (get active_layer_->HighResTiling()->AllTilesForTesting(), and
print tile->tiling_i_index() and tile->tiling_j_index() to see how they look on
a 2d grid)

[sohanjg, 2015/05/08 13:26:09]

Yes, you are right. My mistake.
The tile x,y does show it is a 3X12 layout.

+  EXPECT_EQ(36u, active_layer_->HighResTiling()->AllTilesForTesting().size());
 
   // Visible viewport should be covered by 4 tiles.  No other
   // tiles should be required for activation.