Send empty video frames when screen is not changing.

Send empty video frames when screen is not changing.

Before r272790 the host was sending empty packets 30 times a second
when nothing is changing on the screen. After that change and r281677
the frequency was decreased to 5 frame per second. It looks like those
changes degraded performance for some users. This change partially
restores previous behavior. Empty packets are send for each captured
frame even when nothing is changing on the screen. Keep-alive packets
are sent every 200ms when there are no other video packets, e.g. when
the video stream is paused.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=290028

[Jamie, 2014-08-15 19:14:52]

lgtm

[Sergey Ulanov, 2014-08-15 19:16:25]

The CQ bit was checked by sergeyu@chromium.org

[commit-bot: I haz the power, 2014-08-15 19:18:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sergeyu@chromium.org/477103004/1

[commit-bot: I haz the power, 2014-08-15 22:23:23]

Committed patchset #1 (1) as 290028