Use isFinishedParsingChildren() in HTMLSelectElement

Remove HTMLSelectElement::m_isParsingInProgress and
HTMLSelectElement::isParsingInProgress().

There is no runtime change, as isFinishedParsingChildren() is true per
default, even for script-created elements, and only returns false
between the beginParsingChildren() and finishParsingChildren().

BUG=341414

Review URL: https://codereview.chromium.org/146973006

git-svn-id: svn://svn.chromium.org/blink/trunk@166637 bbb929c8-8fbe-4397-9dbb-9b2b20218538