HTMLTableSectionElement.insertRow()'s argument default value should be -1

HTMLTableSectionElement.insertRow()'s argument default value should be -1, not
0, as per the specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/tabular-data.html#the-tbody-element

The new behavior matches the one of Firefox 29 and IE11.

This CL also cleans up the implementation of HTMLTableSectionElement::insertRow()
a bit.

R=arv@chromium.org, tkent@chromium.org
BUG=369803
TEST=fast/dom/HTMLTableSectionElement/insertRow-default-argument.html

Review URL: https://codereview.chromium.org/263193002

git-svn-id: svn://svn.chromium.org/blink/trunk@173339 bbb929c8-8fbe-4397-9dbb-9b2b20218538