ObjC: Add missing _lastDrawnFrame assignments

Currently there are several checks against _lastDrawnFrame in RTCEAGLVideoView.mm but this variable is not assigned anywhere. Seems like it was missed in 13941912b1 during work on injecting custom shaders.

Bug: webrtc:9133
Change-Id: Ie979a63de343e7253e4b4e70e3b98ffb0880af04
Reviewed-on: https://webrtc-review.googlesource.com/68720
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22819}
diff --git a/AUTHORS b/AUTHORS
index 6ae4803..9394364 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -60,6 +60,7 @@
 Hans Knoechel <hans@hans-knoechel.de>
 Korniltsev Anatoly <korniltsev.anatoly@gmail.com>
 Todd Wong <todd.wong.ndq@gmail.com>
+Maxim Pavlov <pavllovmax@gmail.com>
 
 &yet LLC <*@andyet.com>
 Agora IO <*@agora.io>