cc: Add some unit tests for readbacks outside of the viewport.

cc/trees/layer_tree_host_pixeltest_readback.cc

Index: cc/trees/layer_tree_host_pixeltest_readback.cc
diff --git a/cc/trees/layer_tree_host_pixeltest_readback.cc b/cc/trees/layer_tree_host_pixeltest_readback.cc
index bd6e217969be82bda18d6dbf48dc1ccec6714142..9b5faddc1bd0476575d9fc824fa203c9be52df8f 100644
--- a/cc/trees/layer_tree_host_pixeltest_readback.cc
+++ b/cc/trees/layer_tree_host_pixeltest_readback.cc
@@ -816,6 +816,28 @@ TEST_F(LayerTreeHostReadbackViaCompositeAndReadbackPixelTest,
                                      "green_small_with_blue_corner.png")));
 }
 
+TEST_F(LayerTreeHostReadbackPixelTest, ReadbackNonRootLayerOutsideViewport) {
+  scoped_refptr<SolidColorLayer> background = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorWHITE);
+
+  scoped_refptr<SolidColorLayer> green = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorGREEN);
+  // Only the top left quarter of the layer is inside the viewport, so the
+  // blue layer is entirely outside.
+  green->SetPosition(gfx::Point(100, 100));
+  background->AddChild(green);
+
+  scoped_refptr<SolidColorLayer> blue = CreateSolidColorLayer(
+      gfx::Rect(150, 150, 50, 50), SK_ColorBLUE);
+  green->AddChild(blue);
+
+  RunPixelTestWithReadbackTarget(GL_WITH_DEFAULT,
+                                 background,
+                                 green.get(),
+                                 base::FilePath(FILE_PATH_LITERAL(
+                                     "green_with_blue_corner.png")));
+}
+
 }  // namespace
 }  // namespace cc