Adding test for SingleNalUnit mode

Test enables single-nalu mode, sets limit for nalu lenght and verifies
that encoder follows that limit.
I found that QP jumps significantly when the mode is enabled. In result
encoder might produce 4kbyte and 0.4kbyte frames back-to-back. But it
seems that happens only to couple of frames in the beginning. This
caused test to fail with default RC thresholds. To bypass this I
increased frame size mismatch threshold from 20 to 30%. This should be
Ok considering single-nalu mode is rare.

BUG=webrtc:8070

Review-Url: https://codereview.webrtc.org/3014623002
Cr-Commit-Position: refs/heads/master@{#20023}

modules/video_coding/BUILD.gn

@@ -377,6 +377,7 @@ if (rtc_include_tests) {
     }
 
     deps = [
+      ":codec_globals_headers",
       ":video_coding",
       ":video_coding_utility",
       ":webrtc_vp8",