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..8f5f5c01a60ff72ec271209683a85dc3c8dbf9fe 100644
--- a/content/test/data/media/webrtc_test_utilities.js
+++ b/content/test/data/media/webrtc_test_utilities.js
@@ -66,9 +66,14 @@ function detectVideo(videoElementName, predicate, callback) {
   var width = VIDEO_TAG_WIDTH;
   var height = VIDEO_TAG_HEIGHT;
   var videoElement = $(videoElementName);
-  var canvas = $(videoElementName + '-canvas');
   var oldPixels = [];
+  var startTimeMs = new Date().getTime();
   var waitVideo = setInterval(function() {
+    var canvas = $(videoElementName + '-canvas');
+    if (canvas == null) {
+      console.log('Waiting for ' + videoElementName + '-canvas' + ' to appear');
+      return;
+    }
     var context = canvas.getContext('2d');
     context.drawImage(videoElement, 0, 0, width, height);
     var pixels = context.getImageData(0, 0 , width, height / 3).data;
@@ -82,6 +87,11 @@ function detectVideo(videoElementName, predicate, callback) {
       callback(videoElement.videoWidth, videoElement.videoHeight);
     }
     oldPixels = pixels;
+    var elapsedTime = new Date().getTime() - startTimeMs;
+    if (elapsedTime > 3000) {
+      startTimeMs = new Date().getTime();
+      console.log('Still waiting for video to satisfy ' + predicate.toString());
+    }
   }, 200);
 }
 
@@ -187,17 +197,22 @@ function isVideoPlaying(pixels, previousPixels) {
 }
 
 function isVideoBlack(pixels) {
-  for (var i = 0; i < pixels.length; i++) {
+  var threshold = 20;
+  var accumulatedLuma = 0;
+  for (var i = 4; i < pixels.length; i += 4) {
     // |pixels| is in RGBA. Ignore the alpha channel.

[kjellander_chromium, 2015/05/19 09:10:01]

I'm still missing the explanation on why we ignore the first pixel. I would find
it confusing to read the for loop starting at element 4 in the array if I didn't
know the reason behind it.
I guess the comment could go before the for loop also.

[phoglund_chromium, 2015/05/19 09:37:08]

Oh sorry, I misread your comment and thought you were talking about why we were
ignoring the alpha channel. I have no idea why we would ignore the first pixel,
so I'll just change that to 0 (I assume that must be a bug).

[kjellander_chromium, 2015/05/19 10:06:22]

Hmm, it sounds a bit risky to just assume that's a bug in YUV conversion that
has been fixed. 
I suggest ignoring the first pixel here and follow up with rileya@ who wrote
https://codereview.chromium.org/601083002 to see if it can be removed (or just
do that in a separate CL after committing this).

It's more important to get this back running than fixing two things in the same
CL.

-    // 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) {
+    accumulatedLuma += rec702Luma_(pixels[i], pixels[i + 1], pixels[i + 2]);
+    if (accumulatedLuma > threshold * i / 4)
       return false;
-    }
   }
   return true;
 }
 
+function rec702Luma_(r, g, b) {
+  // Use Luma as in Rec. 709: Y′709 = 0.2126R + 0.7152G + 0.0722B;

[kjellander_chromium, 2015/05/19 09:10:01]

Can you move this up to be function documentation instead?
Maybe also link to http://en.wikipedia.org/wiki/Rec._709.

[phoglund_chromium, 2015/05/19 09:37:08]

Done.

+  return 0.2126 * r + 0.7152 * g + 0.0722 * b;
+}
+
 // This function matches |left| and |right| and fails the test if the
 // values don't match using normal javascript equality (i.e. the hard
 // types of the operands aren't checked).