Use ::cue for VTT UA styles.

third_party/WebKit/Source/core/css/mediaControls.css

Index: third_party/WebKit/Source/core/css/mediaControls.css
diff --git a/third_party/WebKit/Source/core/css/mediaControls.css b/third_party/WebKit/Source/core/css/mediaControls.css
index c8b17dd34baa03d91dfa66b24ab908f151648a9a..595baf18b886d47e015659115f3413d9d20adf5b 100644
--- a/third_party/WebKit/Source/core/css/mediaControls.css
+++ b/third_party/WebKit/Source/core/css/mediaControls.css
@@ -410,14 +410,14 @@ 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;
 }