Initialize svc_drop_frame in vp9 wrapper.
Initialize svc_drop_frame in vp9 wrapper.

Thus we don't need to initialize new members added to the structure
in the future.

Bug: None
Change-Id: Id9f5b127c224660f3016973261045b4231a617c1
Reviewed-on: https://webrtc-review.googlesource.com/79080
Reviewed-by: Sergey Silkin <fc631354f276613a5410ea259cf4618b2846d39b@webrtc.org>
Reviewed-by: Rasmus Brandt <9545de3a34b925a736ae2b5ee593f3033f743a28@webrtc.org>
Commit-Queue: Rasmus Brandt <9545de3a34b925a736ae2b5ee593f3033f743a28@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#23404}
ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc