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::DidChangeFocus(bool has_focus) {
  DCHECK(plugin_task_runner_->BelongsToCurrentThread());

  if (!IsConnected())
    return;

  input_handler_.DidChangeFocus(has_focus);
  if (mouse_locker_)
    mouse_locker_->DidChangeFocus(has_focus);
}

void ChromotingInstance::DidChangeFocus(bool has_focus) {
  DCHECK(plugin_task_runner_->BelongsToCurrentThread());

  if (!IsConnected())
    return;

  input_handler_.DidChangeFocus(has_focus);
  if (mouse_locker_)
    mouse_locker_->DidChangeFocus(has_focus);
}
