Revert of Simplify LoadableTextTrack::trackElementIndex

third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp

Index: third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
diff --git a/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp b/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
index a8078faff80667687abc31e791aa07083cc835d5..c61c4fd9a6b425d5b5dad9a559e28296d4db28e3 100644
--- a/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
+++ b/third_party/WebKit/Source/core/html/track/LoadableTextTrack.cpp
@@ -35,7 +35,6 @@
     : TextTrack(subtitlesKeyword(), emptyAtom, emptyAtom, emptyAtom, TrackElement)
     , m_trackElement(track)
 {
-    DCHECK(m_trackElement);
 }
 
 LoadableTextTrack::~LoadableTextTrack()
@@ -44,6 +43,7 @@
 
 bool LoadableTextTrack::isDefault() const
 {
+    ASSERT(m_trackElement);
     return m_trackElement->fastHasAttribute(HTMLNames::defaultAttr);
 }
 
@@ -62,15 +62,22 @@
     }
 }
 
-size_t LoadableTextTrack::trackElementIndex() const
+size_t LoadableTextTrack::trackElementIndex()
 {
-    // Count the number of preceding <track> elements (== the index.)
+    ASSERT(m_trackElement);
+    ASSERT(m_trackElement->parentNode());
+
     size_t index = 0;
-    const HTMLTrackElement* track = m_trackElement;
-    while ((track = Traversal<HTMLTrackElement>::previousSibling(*track)))
+    for (HTMLTrackElement* track = Traversal<HTMLTrackElement>::firstChild(*m_trackElement->parentNode()); track; track = Traversal<HTMLTrackElement>::nextSibling(*track)) {
+        if (!track->parentNode())
+            continue;
+        if (track == m_trackElement)
+            return index;
         ++index;
+    }
+    ASSERT_NOT_REACHED();
 
-    return index;
+    return 0;
 }
 
 DEFINE_TRACE(LoadableTextTrack)