Use FrameSelection::IsDirectional() in TextControlElement

Use FrameSelection::IsDirectional() in TextControlElement
This is done as a preparation of moving member variable is_directional_ of
class SelectionTemplate to class FrameSelection for code health.
BUG=740804

Change-Id: I287ed4b01ffe531c38c09a79f4f90dd5a5abc896
Reviewed-on: https://chromium-review.googlesource.com/589092
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490747}

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

@@ -590,7 +590,7 @@ TextFieldSelectionDirection TextControlElement::ComputeSelectionDirection()
   const SelectionInDOMTree& selection =
       frame->Selection().GetSelectionInDOMTree();
   const Position& start = selection.ComputeStartPosition();
-  return selection.IsDirectional()
+  return frame->Selection().IsDirectional()
              ? (selection.Base() == start ? kSelectionHasForwardDirection
                                           : kSelectionHasBackwardDirection)
              : kSelectionHasNoDirection;