Previous WebrtcVideoEncoderVpx was implementing VideoEncoder interface.
That interface depends on VideoPacket, which is not applicable for
WebRTC case. Added a separate interface that will be used for WebRTC
protocol. The new interface allows to set bandwidth and frame duration
for each frame individually.
Also renamed class previously called WebrtcVideoEncoder to
WebrtcDummyVideoEncoder.

BUG=645656

Review-Url: https://codereview.chromium.org/2329653002
Cr-Commit-Position: refs/heads/master@{#418314}