Skip current_video_frame increment when a frame is dropped

When the encoder decides to drop a frame from encoding, do not
increase the video count. This is an encoder change. It does not
affect the decoder behavior.

This fixes a set of unit test failure cases when the rate control
decides to drop frames.

BUG=aomedia:1434

Change-Id: I6652d7406bed7406c7626d3efc458dfbfe6666ba

av1/encoder/encoder.c

diff --git a/av1/encoder/encoder.c b/av1/encoder/encoder.c
index 82157e4..353c4f5 100644
--- a/av1/encoder/encoder.c
+++ b/av1/encoder/encoder.c
@@ -6007,7 +6007,6 @@
       cm->frame_type != KEY_FRAME) {
     if (av1_rc_drop_frame(cpi)) {
       av1_rc_postencode_update_drop_frame(cpi);
-      ++cm->current_video_frame;
       aom_free(tile_ctxs);
       aom_free(cdf_ptrs);
       return AOM_CODEC_OK;