Replaced is_null() with is_valid in SurfaceId and related classes.

cc/layers/surface_layer_unittest.cc

Index: cc/layers/surface_layer_unittest.cc
diff --git a/cc/layers/surface_layer_unittest.cc b/cc/layers/surface_layer_unittest.cc
index 3ed3d7e8e6127bdb29396d63d608c771aa87c87b..3f381ce42f928c956abb7dae9b3d5777139ed199 100644
--- a/cc/layers/surface_layer_unittest.cc
+++ b/cc/layers/surface_layer_unittest.cc
@@ -91,7 +91,7 @@ TEST_F(SurfaceLayerTest, MultipleFramesOneSurface) {
   layer_tree_host2->SetRootLayer(layer2);
 
   // Layers haven't been removed, so no sequence should be satisfied.
-  EXPECT_TRUE(blank_change.is_null());
+  EXPECT_FALSE(blank_change.is_valid());
 
   SurfaceSequence expected1(FrameSinkId(1, 1), 1u);
   SurfaceSequence expected2(FrameSinkId(2, 2), 1u);
@@ -218,7 +218,7 @@ class SurfaceLayerSwapPromiseWithDraw : public SurfaceLayerSwapPromise {
     EXPECT_EQ(1u, required_set_.size());
     // Sequence should have been satisfied through Swap, not with the
     // callback.
-    EXPECT_TRUE(satisfied_sequence_.is_null());
+    EXPECT_FALSE(satisfied_sequence_.is_valid());
   }
 };