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::PostLegacyJsonMessage(
    const std::string& method,
    scoped_ptr<base::DictionaryValue> data) {
  scoped_ptr<base::DictionaryValue> message(new base::DictionaryValue());
  message->SetString("method", method);
  message->Set("data", data.release());

  std::string message_json;
  base::JSONWriter::Write(message.get(), &message_json);
  PostMessage(pp::Var(message_json));
}

void ChromotingInstance::PostLegacyJsonMessage(
    const std::string& method,
    scoped_ptr<base::DictionaryValue> data) {
  scoped_ptr<base::DictionaryValue> message(new base::DictionaryValue());
  message->SetString("method", method);
  message->Set("data", data.release());

  std::string message_json;
  base::JSONWriter::Write(message.get(), &message_json);
  PostMessage(pp::Var(message_json));
}
