Add a stat to the smoothness benchmark for avg number of missing tiles.

cc/tiled_layer_impl.cc

Index: cc/tiled_layer_impl.cc
diff --git a/cc/tiled_layer_impl.cc b/cc/tiled_layer_impl.cc
index cfd22437f71534c3a777e62bb7c509a6b8020b0e..1ee5953f591701e5518196f676737709f02aab22 100644
--- a/cc/tiled_layer_impl.cc
+++ b/cc/tiled_layer_impl.cc
@@ -160,11 +160,13 @@ void TiledLayerImpl::appendQuads(QuadSink& quadSink, AppendQuadsData& appendQuad
 
                     scoped_ptr<CheckerboardDrawQuad> checkerboardQuad = CheckerboardDrawQuad::Create();
                     checkerboardQuad->SetNew(sharedQuadState, tileRect, checkerColor);
-                    appendQuadsData.hadMissingTiles |= quadSink.append(checkerboardQuad.PassAs<DrawQuad>(), appendQuadsData);
+                    if (quadSink.append(checkerboardQuad.PassAs<DrawQuad>(), appendQuadsData))
+                        appendQuadsData.numMissingTiles++;
                 } else {
                     scoped_ptr<SolidColorDrawQuad> solidColorQuad = SolidColorDrawQuad::Create();
                     solidColorQuad->SetNew(sharedQuadState, tileRect, backgroundColor());
-                    appendQuadsData.hadMissingTiles |= quadSink.append(solidColorQuad.PassAs<DrawQuad>(), appendQuadsData);
+                    if (quadSink.append(solidColorQuad.PassAs<DrawQuad>(), appendQuadsData))
+                        appendQuadsData.numMissingTiles++;
                 }
                 continue;
             }