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::SetCapabilities(const std::string& capabilities) {
  scoped_ptr<base::DictionaryValue> data(new base::DictionaryValue());
  data->SetString("capabilities", capabilities);
  PostLegacyJsonMessage("setCapabilities", data.Pass());
}

void ChromotingInstance::SetCapabilities(const std::string& capabilities) {
  scoped_ptr<base::DictionaryValue> data(new base::DictionaryValue());
  data->SetString("capabilities", capabilities);
  PostLegacyJsonMessage("setCapabilities", data.Pass());
}
