Unfocused TEXTAREA element should not accept editing.

The normal unfocusing step clears selection. But unfocusing caused by
display:none style didn't clear selection because:
 - It used Document::setFocusedElement. It bypassed FocusController.
 - Even if it used it, clearSelectionIfNeeded() in FocusController didn't
    clear selection because selection.rootEditableElement() was null in
    this case.

This CL changes the code so that:
 - We use Element::blur(), which uses FocusController::setFocusedElement.
 - clearSelectionIfNeeded() uses enclosingTextFormControl() to check
    if the selection is in INPUT/TEXTAREA instead of
    rootEditableElement(), which depends on renderers.

BUG=364309
TEST=automated

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=172121

Review URL: https://codereview.chromium.org/246073004

git-svn-id: svn://svn.chromium.org/blink/trunk@172318 bbb929c8-8fbe-4397-9dbb-9b2b20218538