Pass captured frames with alpha content to WebRTC

This is a followup to https://webrtc-review.googlesource.com/c/src/+/21130.
Since we have alpha containers for WebRTC now, we no longer need to drop
the alpha planes before sending captured frames.

BUG=webrtc:7671
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Change-Id: Ida2ad85770ba42889582fccffc1e52ec2c100057
Reviewed-on: https://chromium-review.googlesource.com/768308


Commit-Queue: Emircan Uysaler <emircan@chromium.org>
Reviewed-by: Christian Fremerey <chfremer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518289}