Use ::cue for VTT UA styles.

third_party/WebKit/Source/core/css/mediaControlsNew.css

Index: third_party/WebKit/Source/core/css/mediaControlsNew.css
diff --git a/third_party/WebKit/Source/core/css/mediaControlsNew.css b/third_party/WebKit/Source/core/css/mediaControlsNew.css
index 4c33ef01876c25496dac65cf12459fc06545f48e..8b82dbf38019b6c79307403fbbbfe9c77a08c65f 100644
--- a/third_party/WebKit/Source/core/css/mediaControlsNew.css
+++ b/third_party/WebKit/Source/core/css/mediaControlsNew.css
@@ -431,14 +431,14 @@ video::cue(:future) {
     color: gray;
 }
 
-video::-webkit-media-text-track-container b {
+video::cue(b) {
     font-weight: bold;
 }
 
-video::-webkit-media-text-track-container u {
+video::cue(u) {
     text-decoration: underline;
 }
 
-video::-webkit-media-text-track-container i {
+video::cue(i) {
     font-style: italic;
 }