Bug 1556079 - part1 : hide cue if we can't find a good place to put it fully inside the rendering area. r=heycam

According to the spec 7.2.10.17 [1], if we have tried both direction and there is no place to put the cue inside the rendering area without overlapping with other cues or the boundary of rendering area, then we have to discard all CSS boxes, which means that we should not display this cue.

[1] https://www.w3.org/TR/webvtt1/#processing-cue-settings

Differential Revision: https://phabricator.services.mozilla.com/D33729

--HG--
extra : moz-landing-system : lando

dom/media/webvtt/vtt.jsm

@@ -1263,6 +1263,9 @@ XPCOMUtils.defineLazyPreferenceGetter(this, "DEBUG_LOG",
             boxPositions.push(cueBox);
             LOG(`cue ${i}, ` + cueBox.getBoxInfoInChars());
           }
+        } else {
+          LOG(`can not find a proper position to place cue ${i}`);
+          rootOfCues.removeChild(styleBox.div);
         }
       }
     } else {