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}
bool ChromotingInstance::IsConnected() {
  return client_ &&
         (client_->connection_state() == protocol::ConnectionToHost::CONNECTED);
}

bool ChromotingInstance::IsConnected() {
  return client_ &&
         (client_->connection_state() == protocol::ConnectionToHost::CONNECTED);
}
