Add MediaControlTimelineElement FIXME for "timeline offset"

Source/core/html/shadow/MediaControlElements.cpp

Index: Source/core/html/shadow/MediaControlElements.cpp
diff --git a/Source/core/html/shadow/MediaControlElements.cpp b/Source/core/html/shadow/MediaControlElements.cpp
index a6160fe088cae309d0b5ed36b46a924ade9e6480..60dad5dd454311941244cb4f757077c6438f6cc6 100644
--- a/Source/core/html/shadow/MediaControlElements.cpp
+++ b/Source/core/html/shadow/MediaControlElements.cpp
@@ -374,6 +374,8 @@ void MediaControlTimelineElement::defaultEventHandler(Event* event)
 
     double time = value().toDouble();
     if (event->type() == EventTypeNames::input) {
+        // FIXME: This will need to take the timeline offset into consideration
+        // once that concept is supported, see https://crbug.com/312699
         if (mediaElement().controller())
             mediaElement().controller()->setCurrentTime(time, IGNORE_EXCEPTION);
         else