Add basic versioning check to Chromoting's plugin/JS code.

remoting/client/plugin/chromoting_scriptable_object.cc

Index: remoting/client/plugin/chromoting_scriptable_object.cc
diff --git a/remoting/client/plugin/chromoting_scriptable_object.cc b/remoting/client/plugin/chromoting_scriptable_object.cc
index 106141fbcb59beceb57ad3f654e65e2f9ce726b4..e998ff61bf7110f329d78cb93b4200e28e6cc6df 100644
--- a/remoting/client/plugin/chromoting_scriptable_object.cc
+++ b/remoting/client/plugin/chromoting_scriptable_object.cc
@@ -18,6 +18,8 @@ namespace remoting {
 
 namespace {
 
+const char kApiVersionAttribute[] = "apiVersion";
+const char kApiMinVersionAttribute[] = "apiMinVersion";
 const char kConnectionInfoUpdate[] = "connectionInfoUpdate";
 const char kDebugInfo[] = "debugInfo";
 const char kDesktopHeight[] = "desktopHeight";
@@ -48,6 +50,14 @@ void ChromotingScriptableObject::Init() {
   // Property addition order should match the interface description at the
   // top of chromoting_scriptable_object.h.
 
+  // Plugin API version.
+  // This should be incremented whenever the API interface changes.
+  AddAttribute(kApiVersionAttribute, Var(1));
+
+  // This should be updated whenever we remove support for an older version
+  // of the API.
+  AddAttribute(kApiMinVersionAttribute, Var(1));
+
   // Connection status.
   AddAttribute(kStatusAttribute, Var(STATUS_UNKNOWN));