Move Editor::Transpose() out of Editor class

This patch moves |Editor::Transpose()| out of |Editor| class as preparation of
expanding it into |ExecutTranspose()| in "EditorCommand.cpp" to make |Editor|
class simpler for improving code health.

Following patch will expand |Transpose()| into |ExecutTranspose()|.

Bug: 672405
Change-Id: Icde253623f31813d2b4517c4da7d4798bd5fadf6
Reviewed-on: https://chromium-review.googlesource.com/583880
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489518}
static String ValueStyle(LocalFrame& frame, CSSPropertyID property_id) {
  frame.GetDocument()->UpdateStyleAndLayoutIgnorePendingStylesheets();

  return frame.GetEditor().SelectionStartCSSPropertyValue(property_id);
}

static String ValueStyle(LocalFrame& frame, CSSPropertyID property_id) {
  frame.GetDocument()->UpdateStyleAndLayoutIgnorePendingStylesheets();

  return frame.GetEditor().SelectionStartCSSPropertyValue(property_id);
}
