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}
PepperVideoRenderer3D::FrameDecodeTimestamp::FrameDecodeTimestamp(
    uint32_t frame_id,
    base::TimeTicks decode_started_time)
    : frame_id(frame_id), decode_started_time(decode_started_time) {
}

PepperVideoRenderer3D::FrameDecodeTimestamp::FrameDecodeTimestamp(
    uint32_t frame_id,
    base::TimeTicks decode_started_time)
    : frame_id(frame_id), decode_started_time(decode_started_time) {
}
