Add a setDebugDirtyRegion() feature to the client.

Calling remoting.clientSession.setDebugDirtyRegion(true) enables
rendering of each frame's dirty region with an purple, translucent
overlay.

Currently the dirty region is re-rendered immediately for each frame,
with no linger nor fade-out behaviour.

BUG=427659

Review URL: https://codereview.chromium.org/932013002

Cr-Commit-Position: refs/heads/master@{#317496}
void ChromotingInstance::PostChromotingMessage(const std::string& method,
                                               const pp::VarDictionary& data) {
  pp::VarDictionary message;
  message.Set(pp::Var("method"), pp::Var(method));
  message.Set(pp::Var("data"), data);
  PostMessage(message);
}

void ChromotingInstance::PostChromotingMessage(const std::string& method,
                                               const pp::VarDictionary& data) {
  pp::VarDictionary message;
  message.Set(pp::Var("method"), pp::Var(method));
  message.Set(pp::Var("data"), data);
  PostMessage(message);
}
