cc: Make LayerTreeHostImpl::SwapBuffers not virtual.

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 8ff9ad970c5f0e0d8b2e7fffe77a3a9a69206a51..b806833f6c3f87fe88b8054b7056367cf78a934c 100644
--- a/cc/layers/surface_layer_unittest.cc
+++ b/cc/layers/surface_layer_unittest.cc
@@ -191,8 +191,7 @@ class SurfaceLayerSwapPromiseWithDraw : public SurfaceLayerSwapPromise {
     }
   }
 
-  void SwapBuffersOnThread(LayerTreeHostImpl* host_impl, bool result) override {
-    EXPECT_TRUE(result);

[danakj, 2016/07/13 21:50:17]

This function only happens now if swap happened, so it running at all implies
this true.

+  void SwapBuffersCompleteOnThread() override {
     std::vector<uint32_t>& satisfied =
         output_surface()->last_sent_frame()->metadata.satisfies_sequences;
     EXPECT_LE(satisfied.size(), 1u);