Replace redundant <track> runtime checks with asserts

Source/core/html/HTMLMediaElement.cpp

Index: Source/core/html/HTMLMediaElement.cpp
diff --git a/Source/core/html/HTMLMediaElement.cpp b/Source/core/html/HTMLMediaElement.cpp
index ef33ad5a05d2644c3abb38ea7856bdbb08f9d5a1..1c40028bd22265f679a19ed79e003d5204c0a24e 100644
--- a/Source/core/html/HTMLMediaElement.cpp
+++ b/Source/core/html/HTMLMediaElement.cpp
@@ -2535,8 +2535,7 @@ void HTMLMediaElement::removeAllInbandTracks()
 
 PassRefPtr<TextTrack> HTMLMediaElement::addTextTrack(const String& kind, const String& label, const String& language, ExceptionState& es)
 {
-    if (!RuntimeEnabledFeatures::videoTrackEnabled())
-        return 0;
+    ASSERT(RuntimeEnabledFeatures::videoTrackEnabled());
 
     // 4.8.10.12.4 Text track API
     // The addTextTrack(kind, label, language) method of media elements, when invoked, must run the following steps:
@@ -2572,8 +2571,7 @@ PassRefPtr<TextTrack> HTMLMediaElement::addTextTrack(const String& kind, const S
 
 TextTrackList* HTMLMediaElement::textTracks()
 {
-    if (!RuntimeEnabledFeatures::videoTrackEnabled())
-        return 0;
+    ASSERT(RuntimeEnabledFeatures::videoTrackEnabled());
 
     if (!m_textTracks)
         m_textTracks = TextTrackList::create(this);