Decide on IsFinishedParsingChilden for flag name

It seems isFinishedParsingChildren was added because isParsingChildrenFinished is protected in Node.
However we can just choose one name in Node and declare the chosen Node method public in Element.

Review URL: https://codereview.chromium.org/149243006

git-svn-id: svn://svn.chromium.org/blink/trunk@166171 bbb929c8-8fbe-4397-9dbb-9b2b20218538