Add stable id to PaintImage.

cc/trees/layer_tree_host_pixeltest_blending.cc

Index: cc/trees/layer_tree_host_pixeltest_blending.cc
diff --git a/cc/trees/layer_tree_host_pixeltest_blending.cc b/cc/trees/layer_tree_host_pixeltest_blending.cc
index e4d95bc361ec74d39ed7dcf097cd48b31913ff5a..656d2922e027b269fa81f5375b6a3535e048f0f7 100644
--- a/cc/trees/layer_tree_host_pixeltest_blending.cc
+++ b/cc/trees/layer_tree_host_pixeltest_blending.cc
@@ -146,7 +146,8 @@ class LayerTreeHostBlendingPixelTest : public LayerTreeHostPixelResourceTest {
     scoped_refptr<PictureImageLayer> layer = PictureImageLayer::Create();
     layer->SetIsDrawable(true);
     layer->SetBounds(gfx::Size(width, height));
-    layer->SetImage(PaintImage(backing_store->makeImageSnapshot()));
+    layer->SetImage(PaintImage(PaintImage::GetNextId(),
+                               backing_store->makeImageSnapshot()));
     return layer;
   }
 
@@ -168,7 +169,8 @@ class LayerTreeHostBlendingPixelTest : public LayerTreeHostPixelResourceTest {
                                       bounds.width() - kMaskOffset * 2,
                                       bounds.height() - kMaskOffset * 2),
                      paint);
-    mask->SetImage(PaintImage(surface->makeImageSnapshot()));
+    mask->SetImage(
+        PaintImage(PaintImage::GetNextId(), surface->makeImageSnapshot()));
     layer->SetMaskLayer(mask.get());
   }