Measure encoding time on encode callbacks.

Permits measuring encoding time even when performed on another thread,
typically for hardware encoding, instead of assuming that encoding is
blocking the calling thread.

Permitted encoding time is increased for hardware encoders since they
can be timed to keep 30fps, for instance, without indicating overload.

Merges EncodingTimeObserver into EncodedFrameObserver to have one post-encode
callback.

BUG=webrtc:5042, webrtc:5132
R=asapersson@webrtc.org, mflodman@webrtc.org

Review URL: https://codereview.webrtc.org/1569853002 .

Cr-Commit-Position: refs/heads/master@{#11499}

webrtc/modules/video_coding/video_coding_impl.h

@@ -31,8 +31,6 @@
 
 namespace webrtc {
 
-class EncodedFrameObserver;
-
 namespace vcm {
 
 class VCMProcessTimer {