Adds a TransportDIB::Id value that is explicitly invalid and use it when compositing

app/surface/transport_dib_mac.cc

Index: app/surface/transport_dib_mac.cc
diff --git a/app/surface/transport_dib_mac.cc b/app/surface/transport_dib_mac.cc
index 8ea81ae26a0aa9732d69e89c04a06118a466bda0..c0dd45716824fb9e776733a399cf9e4da40c7999 100644
--- a/app/surface/transport_dib_mac.cc
+++ b/app/surface/transport_dib_mac.cc
@@ -51,10 +51,15 @@ TransportDIB* TransportDIB::CreateWithHandle(Handle handle) {
 }
 
 // static
-bool TransportDIB::is_valid(Handle dib) {
+bool TransportDIB::is_valid_handle(Handle dib) {
   return dib.fd >= 0;
 }
 
+// static
+bool TransportDIB::is_valid_id(Id id) {
+  return id != 0;
+}
+
 skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
   if (!memory() && !Map())
     return NULL;
@@ -65,7 +70,7 @@ skia::PlatformCanvas* TransportDIB::GetPlatformCanvas(int w, int h) {
 }
 
 bool TransportDIB::Map() {
-  if (!is_valid(handle()))
+  if (!is_valid_handle(handle()))
     return false;
   if (memory())
     return true;