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

blimp/client/core/compositor/blimp_compositor.cc

Index: blimp/client/core/compositor/blimp_compositor.cc
diff --git a/blimp/client/core/compositor/blimp_compositor.cc b/blimp/client/core/compositor/blimp_compositor.cc
index 9fa2d525a905e470888d9d2ab5a558b4898e8638..b33b2037baf4fa70a87aeaac5368dcd02093337a 100644
--- a/blimp/client/core/compositor/blimp_compositor.cc
+++ b/blimp/client/core/compositor/blimp_compositor.cc
@@ -213,7 +213,7 @@ void BlimpCompositor::SubmitCompositorFrame(cc::CompositorFrame frame) {
       frame.delegated_frame_data->render_pass_list.back().get();
   gfx::Size surface_size = root_pass->output_rect.size();
 
-  if (local_frame_id_.is_null() || current_surface_size_ != surface_size) {
+  if (!local_frame_id_.is_valid() || current_surface_size_ != surface_size) {
     DestroyDelegatedContent();
     DCHECK(layer_->children().empty());
 
@@ -275,7 +275,7 @@ CompositorDependencies* BlimpCompositor::GetEmbedderDeps() {
 }
 
 void BlimpCompositor::DestroyDelegatedContent() {
-  if (local_frame_id_.is_null())
+  if (!local_frame_id_.is_valid())
     return;
 
   // Remove any references for the surface layer that uses this