cc: Update SpreadForStdDeviation to 3*sigma, and update tests.

cc/trees/layer_tree_host_common_unittest.cc

Index: cc/trees/layer_tree_host_common_unittest.cc
diff --git a/cc/trees/layer_tree_host_common_unittest.cc b/cc/trees/layer_tree_host_common_unittest.cc
index bad835b90f8aeacb2a8606c39dccd34df32c3d05..a6b40e409eaeddcbd42b1ed2b05bc2e193d864ad 100644
--- a/cc/trees/layer_tree_host_common_unittest.cc
+++ b/cc/trees/layer_tree_host_common_unittest.cc
@@ -1407,7 +1407,7 @@ TEST_F(LayerTreeHostCommonTest, RenderSurfaceListForFilter) {
   ASSERT_TRUE(parent->render_surface());
   EXPECT_EQ(2U, parent->render_surface()->layer_list().size());
   EXPECT_EQ(4U, render_surface_layer_list.size());
-  EXPECT_EQ(gfx::RectF(-29, -29, 158, 158),
+  EXPECT_EQ(gfx::RectF(-30, -30, 160, 160),
             parent->render_surface()->DrawableContentRect());
 }