Turn off per-layer frame dropping.

Per-layer frame dropping was recently implemented in VP9 SVC encoder
and set as default mode.

This disables per-layer frame dropping in WebRTC VP9 encoder wrapper
since receiver (jitter buffer) can't handle such drops yet.

Bug: none
Change-Id: Iad5491abf1e3fc1bccfe44eb7276ff6363176029
Reviewed-on: https://webrtc-review.googlesource.com/66460
Reviewed-by: Marco Paniconi <marpan@google.com>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22721}