Call this function when the user drags the mouse into the web view (before
calling DragTargetDragOver/DragTargetLeave/DragTargetDrop). |drag_data|
should not contain file contents as this type of data is not allowed to be
dragged into the web view. File contents can be removed using
Call this function when the user drags the mouse into the web view (before calling DragTargetDragOver/DragTargetLeave/DragTargetDrop). |drag_data| should not contain file contents as this type of data is not allowed to be dragged into the web view. File contents can be removed using