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 PepperVideoRenderer2D::OnSessionConfig(
     const protocol::SessionConfig& config) {
   DCHECK(CalledOnValidThread());

  software_video_renderer_->OnSessionConfig(config);
  AllocateBuffers();
}

 void PepperVideoRenderer2D::OnSessionConfig(
     const protocol::SessionConfig& config) {
   DCHECK(CalledOnValidThread());

  software_video_renderer_->OnSessionConfig(config);
  AllocateBuffers();
}
