Use and add layout tests for BCP 47 language tag syntax validation.

Source/modules/mediasource/TrackDefault.cpp

Index: Source/modules/mediasource/TrackDefault.cpp
diff --git a/Source/modules/mediasource/TrackDefault.cpp b/Source/modules/mediasource/TrackDefault.cpp
index 6485d2eabbbd96dddf868356e8aa0cee64935bd9..3d43756322b9b96cdd5c428c439bee89900a71b7 100644
--- a/Source/modules/mediasource/TrackDefault.cpp
+++ b/Source/modules/mediasource/TrackDefault.cpp
@@ -10,6 +10,7 @@
 #include "core/html/track/AudioTrack.h"
 #include "core/html/track/TextTrack.h"
 #include "core/html/track/VideoTrack.h"
+#include "public/platform/Platform.h"
 
 namespace blink {
 
@@ -40,7 +41,10 @@ TrackDefault* TrackDefault::create(const AtomicString& type, const String& langu
     // When this method is invoked, the user agent must run the following steps:
     // 1. if |language| is not an empty string and |language| is not a BCP 47
     //    language tag, then throw an INVALID_ACCESS_ERR and abort these steps.
-    // FIXME: Implement BCP 47 language tag validation.
+    if (!Platform::current()->isValidLocaleSyntax(language)) {
+        exceptionState.throwDOMException(InvalidAccessError, "Invalid BCP 47 language tag");
+        return nullptr;
+    }
 
     if (type == audioKeyword()) {
         // 2.1. If |type| equals "audio":