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::LogToWebapp(const std::string& message) {
  DCHECK(plugin_task_runner_->BelongsToCurrentThread());

  LOG(ERROR) << message;

#if !defined(OS_NACL)
  ProcessLogToUI(message);
#endif  // !defined(OS_NACL)
}

void ChromotingInstance::LogToWebapp(const std::string& message) {
  DCHECK(plugin_task_runner_->BelongsToCurrentThread());

  LOG(ERROR) << message;

#if !defined(OS_NACL)
  ProcessLogToUI(message);
#endif  // !defined(OS_NACL)
}
