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}
  PendingPacket(scoped_ptr<VideoPacket> packet, const base::Closure& done)
      : packet_(packet.Pass()),
        done_runner_(done) {
  }

  PendingPacket(scoped_ptr<VideoPacket> packet, const base::Closure& done)
      : packet_(packet.Pass()),
        done_runner_(done) {
  }
