Try switching black frame algorithm to a luma-based algorithm.

content/test/data/media/webrtc_test_utilities.js

Index: content/test/data/media/webrtc_test_utilities.js
diff --git a/content/test/data/media/webrtc_test_utilities.js b/content/test/data/media/webrtc_test_utilities.js
index 5fd38113882b7c6bf7f9db219850291da9e4d2c6..3e623df4a12ad9e770ec081dc230ebb446800382 100644
--- a/content/test/data/media/webrtc_test_utilities.js
+++ b/content/test/data/media/webrtc_test_utilities.js
@@ -187,13 +187,14 @@ function isVideoPlaying(pixels, previousPixels) {
 }
 
 function isVideoBlack(pixels) {
-  for (var i = 0; i < pixels.length; i++) {
-    // |pixels| is in RGBA. Ignore the alpha channel.
-    // We allow it to be off by 1, to account for rounding errors in YUV
-    // conversion.
-    if (pixels[i] != 0 && pixels[i] != 1 && (i + 1) % 4 != 0) {
+  var threshold = 20;
+  var accumulatedLuma = 0;
+  for (var i = 4; i < pixels.length; i += 4) {

[kjellander_chromium, 2015/05/19 08:08:54]

Please keep the comment about that |pixels| is in RGBA and why we ignore the
first pixel.

[phoglund_chromium, 2015/05/19 09:01:19]

Done.

+    // Use Luma as in Rec. 709: Y′709 = 0.21R + 0.72G + 0.07B;

[kjellander_chromium, 2015/05/19 08:08:54]

I'm new to color space standards, but according to
http://en.wikipedia.org/wiki/Rec._709 "HDTV according to Rec. 709 forms luma
(Y’) using R’G’B’ coefficients 0.2126, 0.7152, and 0.0722".
I guess the decimals are not a big deal, but we might as well use the full
constants.

[phoglund_chromium, 2015/05/19 09:01:20]

Done.

+    accumulatedLuma +=
+        0.21 * pixels[i] + 0.72 * pixels[i + 1] + 0.07 * pixels[i + 2];

[kjellander_chromium, 2015/05/19 08:08:54]

Can you extract what's after += to a function?

[phoglund_chromium, 2015/05/19 09:01:19]

Done.

+    if (accumulatedLuma > threshold * i / 4)
       return false;
-    }
   }
   return true;
 }