Implement seekable() according to the MediaSource specification

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 e40f1e15a0b88b504965cd6cf87442061d418be8..83501f963528b688a2843f831a48733b5a6ecb3b 100644
--- a/Source/core/html/HTMLMediaElement.cpp
+++ b/Source/core/html/HTMLMediaElement.cpp
@@ -3352,6 +3352,9 @@ PassRefPtrWillBeRawPtr<TimeRanges> HTMLMediaElement::seekable() const
     if (!webMediaPlayer())
         return TimeRanges::create();
 
+    if (m_mediaSource)
+        return m_mediaSource->seekable();
+
     return TimeRanges::create(webMediaPlayer()->seekable());
 }