Use a ResizeObserver to determine default font-size for text tracks

third_party/WebKit/Source/core/html/HTMLMediaElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp b/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
index adb58f1315558b7ac69d56e946373ad76c02367a..7ad5bc11304323b5674a7c85fcfa105d45b89753 100644
--- a/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLMediaElement.cpp
@@ -3663,8 +3663,7 @@ TextTrackContainer& HTMLMediaElement::EnsureTextTrackContainer() {
   if (first_child && first_child->IsTextTrackContainer())
     return ToTextTrackContainer(*first_child);
 
-  TextTrackContainer* text_track_container =
-      TextTrackContainer::Create(GetDocument());
+  TextTrackContainer* text_track_container = TextTrackContainer::Create(*this);
 
   // The text track container should be inserted before the media controls,
   // so that they are rendered behind them.