Don't iterate over a 0 mask resource in RenderPassDrawQuad

cc/quads/draw_quad_unittest.cc

Index: cc/quads/draw_quad_unittest.cc
diff --git a/cc/quads/draw_quad_unittest.cc b/cc/quads/draw_quad_unittest.cc
index 3c17644f88190f529f98204a2bc3e320c78f027c..9b5c8fdb90ced5e8d5cbdf233e8a701b0d51ae40 100644
--- a/cc/quads/draw_quad_unittest.cc
+++ b/cc/quads/draw_quad_unittest.cc
@@ -770,6 +770,9 @@ TEST_F(DrawQuadIteratorTest, RenderPassDrawQuad) {
   EXPECT_EQ(mask_resource_id, quad_new->mask_resource_id);
   EXPECT_EQ(1, IterateAndCount(quad_new.get()));
   EXPECT_EQ(mask_resource_id + 1, quad_new->mask_resource_id);
+  quad_new->mask_resource_id = 0;
+  EXPECT_EQ(0, IterateAndCount(quad_new.get()));
+  EXPECT_EQ(0u, quad_new->mask_resource_id);
 }
 
 TEST_F(DrawQuadIteratorTest, SolidColorDrawQuad) {